[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.]
 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