Is the Mitzvah to pray Biblical or Rabbinical?
Personal prayers–Praying in distress: [According to all Poskim] it is a Biblical command to Daven to Hashem in a time of need, when one is in a state of distress and requires Divine assistance. [Furthermore, this is the main intent of prayer, to pray for one’s needs and desires to be fulfilled. Hence those personal prayers which request from Hashem his personal needs carry a special quality that is not found in the set daily prayers. Accordingly, each time one Davens Shacharis, Mincha and Maariv and feels distressed over the exile, he fulfills the Biblical command of prayer according to all. Some Poskim rule that personal prayers carry the same laws and guidelines as does Shmoneh Esrei. One may mention Hashem’s name in these prayers and they may be said any time he desires. It is not necessary to recite them only in Shemoneh Esrei.]
Praying each day irrelevant of distress: Some Poskim rule it is a Biblical obligation, and positive command, to Daven [to Hashem each day, even when one is not in a time of need and does not have any necessary requests]. Other Poskim however rule there is no Biblical obligation to Daven each day and it is rather a Rabbinical obligation. Practically, the main ruling follows the latter opinion that daily prayer is only a Rabbinical obligation. [Nevertheless, even according to the latter opinion, the service of prayer is the foundation of the entire Torah, to know Hashem and recognize His greatness. Likewise, prayer is considered the spinal cord of all the Mitzvos, and serves as the energy and vitality that enliven our Mitzvos and make them effective. It is not counted as one of the 248 positive commands being it is higher than them. Accordingly, irrelevant of the dispute of whether daily prayer is a Biblical obligation, all agree that when one Davens he fulfills a Biblical Mitzvah. Many Mitzvos are possibly fulfilled throughout the prayer such as 1) Belief in Hashem; 2) Achdus Hashem; 3) Ahavas Hashem; 4) Fear of Hashem; 5) Bitachon in Hashem.]
The time of prayer: Even according to the former opinion, the times of prayer is not Biblically mandated and one may pray to fulfill his obligation whenever he chooses. The set times of prayer are merely Rabbinical.
The amount of prayer: Even according to the former opinion, there is no Biblical obligation to Daven more than one time per day [unless one is in a time of need to make a request from Hashem]. When one Davens one time, whether at day or at night, he fulfills his obligation.
The Nussach of prayer: Shachris, Mincha, Maariv: Even according to the former opinion, there is no Biblically required dialect of prayer [and the dialect of prayer is Rabbinical].
 See Admur 106/2; Chinuch Mitzvah 433; Peri Chadash 89; Shagas Aryeh 14; P”M Pesicha Koleles 89; Piskei Dinim Tzemahc Tzedek Rambam TefilaDerech Mitzvosecha Shoresh Mitzvah Hatefila 1; Toras Menachem 27 p. 47; Piskeiy Teshuvos 89/1; Shaareiy Tefila of Heichal Menchame p. 1-17
 M”A 106/2 in name of Semak “It is a Mitzvah to Daven in a time of need”; Ramban Sefer Hamitzvos Hasagos on Mitzvah 5 “However perhaps the intent of the verse “And to serve Him” is to teach us that one is to call onto Hashem in a time of suffering and that our eyes be lifted to Him like the eyes of a slave to his master…” [See Chinuch ibid who writes the Ramban is unsure as to if this is the meaning of the Mitzvah; Liekwise see Chinuch ibid who learsn Ramban to refer even to a private Tzarah, and not just Tzaras Hatzibur; See Igros Moshe 2/25] Chinuch Mitzvah 433 in explanation of Shoresh HaMitzvah “Hashem commanded us to pray in order so we always ask Him for our needs and desires. Aside for receiving our requests we also penetrate our hearts with faith in Hashem and that he oversees everything”; Derech Mitzvosecha ibid
 Likkutei Sichos 29/183; Piskeiy Teshuvos 89/1; Vetzaruch Iyun if this means that according to both Ramban and Ramban this is a Biblical command, or if it means that even according to Ramban it is a Biblical command when in need. The practical ramification is in a case that one already Davened that day, do we say that Davening again in a time of need is a Biblical command even according to Rambam, or only according to Ramban.
 Likkutei Sichos 29/183 “The main aspect of prayer is to effect a change, and such an aspect is not so relevant to the daily prayers but rather specifically in the personal prayers where one requests a real and true salvation from his current state. This is why the Ramban and others argued on the Rambam and held that the command of prayer is not daily, but rather when in need.”
 See Imrei Pinchas 62; Kol Mivbaser; Magen Avraham Balak in name of Besht; Machaneh Yisrael [of Chofetz Chaim] 10; Chazon Ish 23; Piskeiy Teshuvos 89 footnote 19
 Divrei Yoel 139
 Machazik Bracha end of 101
 Chazon Ish ibid
 1st opinion in Admur 106/2; Michaber 106/2 as explained in M”A ibid and P”M 4 M”Z 15, unlike Taz 106/2; Rambam Tefila 1/1 “It is a positive command to Daven each day”; Rif Brachos 17b; Riea Brachos 10b; Semag Asei 19; Chinuch Mitzvah 248 and 433 rules like Rambam; Admur Hazakein in Igros Kodesh p. 34 “Those who say that prayer is Rabbinical have not seen light in their days, as although the dialect and number of prayers is Rabbinical, nevertheless, its main aspect and essence is the foundation of the entire Torah, to know Hashem and recognize His greatness.” See Toras Menachem 27 p. 47 for explanation of this letter of Admur. Admur in Tanya Kuntrus Achron p. 162 “Prayer is an actual Torah obligation]”
Definition of prayer according to this opinion: The definition of the obligation of daily prayer according to the Rambam is to “Supplicate and pray each day, saying the praise of Hashem and then asking Hashem for his needs, and then thanking Hashem for the good he receives.” [Rambam ibid] In truth however, even according to the Rambam, it is not necessary to fulfill the Biblical command for one to perform all three steps of praise, request and thanks, and rather the main Mitzvah is fulfilled through request. [So is evident from Admur 106/2; M”A 106/2; M”B 106/4 and all other Poskim who bring that women fulfill their obligation with the recital of a request from Hashem] Some Poskim however learn the Rambam literally, that one must praise Hashem [and thank Him] to fulfill the Biblical obligation. [See P”M in Pesicha Kolelels and 106 A”A 2 who questions M”A ibid “Praising Hashem is Biblcial”; Magen Goborim 106/3, brought in Shaar Hatziyon 106/5] See Or Liy [of Sdei Chemed] 8; Eretz Tzevi 1/34; Igros Moshe 4/101
 See P”M Pesicha Koleles 89 “Even if one has everything he needs and wants nevertheless he must request for the future”
 The reason: This is learned from the verse “Uliavdo Bechol Levavchem/And you shall serve Him with all your heart”, and [the Sages stated in Taanis 2a] what service is in the heart, prayer. [Admur ibid; M”A 106/2; Rambam ibid] Now, the continuation of that verse states that Hashem will bless our bread and water, and just like bread and water is needed for daily consumption, so too the Mitzvah of serving Hashem in prayer is required daily. [Kiryat Sefer of Mabit on Rambam ibid, brought in P”M Pesicha 89]
 2nd opinion in Admur 106/2 and Likkutei Torah Balak 70c; Rama 55 as explained in P”M 106 A”A 2 [however see P”M 101]; M”A 106/2 that so rule majority of Rishonim; Taz 106/2; Ramban in Hasagos Sefer Hamitzvos 5 “The words of the Rambam are incorrect in my opinion, as certainly the Mitzvah of prayer is not Biblical at all and is rather an act of kindness that Hashem bestowed us”; Rashi Brachos 20b; Tosafus Brachos ibid; Rabbeinu Chananel Brachos ibid; Semak Mitzvah 12; Rashbatz in Zohar Harakia 12; P”M 106 M”Z 2; Ateres Tzevi; Derech Mitzvosecha ibid “Majority of the Moneiy Hamitzvos”
Shabbos and Yom Tov: Some Poskim rule that even according to the Ramban and the Poskim who rule likewise, agree that it is a Biblcial obligation to Daven to Hashem on Shabbos and Yom Tov. [P”M end of Pesicha Koleles and 106 A”A 3; 490 M”Z 2
 The reason: The main aspect of prayer is to effect a change, and such an aspect is not so relevant to the daily prayers but rather specifically in the personal prayers where one requests a real and true salvation from his current state. This is why the Ramban and others argued on the Rambam and held that the command of prayer is not daily, but rather when in need. [Likkutei Sichos 29/183]
 Admur 106/2 and in Likkutei Torah Balak 70c; M”A ibid in name of Poskim; Conclusion of Peri Chadash 89; Conclusion of Shagas Aryeh 14; Tzemach Tzedek in Derech Mitzvosecha ibid that so rule majority of Poskim; See Admur Hazakein in Igros Kodesh p. 34 “Those who say that prayer is Rabbinical have not seen light in their days” and see explanation in Toras Menachem 27 p. 47 [Shaareiy Tefila 1] for how this is not a contradiction
 Admur Hazakein in Igros Kodesh p. 34 as explained in Toras Menachem 27 p. 47; See also Likkutei Torah Balak ibid for lengthy explanation; Explanation: The fact that prayer is Rabbinical is not because it is of less importance than the other Mitzvos, but on the contrary, it is the soul behind all the Mitzvos and hence was not listed as one of the commandments (which are only the body and vessels for the G-dliness.) When we say that prayer is Rabbinical, this only refers to the time and Nussach of prayer. However, the idea of prayer is the root and foundation of all the Biblical commandments. Thus, one who insists that even the essence of prayer is Rabbinical has never seen light in all his days. This means he has yet to recognize the G-dliness drawn through prayer into his Mitzvos. [Rebbe ibid]
 Likkutei Torah Balak ibid; Toras Menachem 27 p. 47; See our Sefer “The Chassidishe Parsha-Torah Or/Likkutei Torah” Mamar Parshas Balak
 Chidushei Hagrach Mebrisk on Rambam Tefila 4/1; See Emek Bracha Nesias Kapayim 1; ; Yabia Omer 3/8; Mishneh Halachos 3/10; Piskeiy Tehsuvos 89/1 footnote 7
 Tanya ibid; Kuntrus Havoda p. 50
 See Chovos Halevavos Shaar Cheshbon Nefesh 3/9
 See Igros Kodsh 14/73
 Admur ibid; Rambam ibid
Praying at night: See the following Poskim regarding if the Biblical command of praying daily can be fulfilled at night according to Rambam: P”M 4 M”Z 15; Ishei Yisrael 7 footnote 8; Piskeiy Teshuvos 89 footnote 2
 Admur ibid; M”A ibid; Rambam ibid
 Admur ibid; M”A ibid; Rambam ibid