Saying “Avraham Zakein” after Hallel:
Some are accustomed to recite on Rosh Chodesh, after Hallel, the verse beginning “Veavraham Zakein” and “Zevadya Yishmereini”. [Practically, so is the Chabad custom.] All this [Avraham Zakein and Zevadya Yishmereini] is recited a total of three times. [One is to say Avraham Zakein and Zevadaya Yishmereini together three times, and not first Avraham Zakein three times and then Zevadya three times. The Kabbalists record the Kabalistic intents that one should have while reciting it, although this matter is omitted by Admur.]
The statement of Veavraham Zakein:
ואברהם זקן בא בימים וה ברך את אברהם בכל.
זבדיה ישמרני ויחייני כן יהי רצון מלפניך אלהים חיים ומלך עולם אשר בידו נפש כל חי אמן
If one did not recite the above statements after Halel, may it be said later on in the day?
The above sources all record to recite the verses specifically after Hallel. It is unclear if the Segula remains if it is recited later on in the day, and if one is allowed to mention the name of the angle, in such a case.
 Siddur Admur; Siddur Rav Shabsi [student of Baal Shem Tov] and Rav Yaakov Kopel [brought in Shaar Hakolel 37:9]; Mishnas Chassidim Rosh Chodesh 2:1 [1700’s]; Sefer Chemdas Yamim [1700’s]; Machberes Hakodesh [Harav Nassan Shapiro-1600’s] Shaar Rosh Chodesh in name of olod manuscript; Kaf Hachaim 422:57; It is omitted in Siddur Harizal and in Peri Eitz Chaim [Shaar Hakolel ibid]
What is the first recorded source of the custom: The Sefarim Chemdas Yamim, Mishnas Chassidim and Machberes Hakodesh are the earliest possible sources for the custom. Historically, the Sefer Machberes Hakodesh is the first written source of this custom, from which seemingly all the other Sefarim listed, recorded it. Thus, irrelevant of the controversy over the Sefer Chemdas Yamim, its source remains authentic and reputable. The fact the Chmedas Yamim decided to record it, does not diminish its nature or authenticity.
 So is the wording of Admur in the Siddur, and it refers to the above Sefarim who recorded the custom. Vetzaruch Iyun why Admur recorded the custom as “some say.” Perhaps Admur questioned the custom for various reasons, as explained in the footnotes here.
 Verbalizing the name of the angel Zevadya: Zevadya is the name of an angel. Some are accustomed not to recite the names Zevadya due to the prohibition in reciting the name of angels. [Custom of Arizal, as brought in Kaf Hachaim 5:10 in name of Shaar Hamitzvos, Nagid Mitzvah, Shelah; Shaar Hakavanos] Hence, they say this word in their mind. Nevertheless, Admur writes “And he should say Zevadya..” in order to emphasize that one is specifically to verbalize it. This allowance is seemingly due to that it is also the name of a person in Tanach. [Shaar Hakolel 37:10; See Nefesh Chaim [Falagi] 32] Alternatively, the prohibition of mentioning the name of an angel only applies if one does so for no reason, while if one does so as part of prayer, it is permitted. [See Kaf Hachaim 5:10 that the Arizal would not mention the angels in middle of a “Drasha”] Some Siddurim however avoid this issue by replacing the name Zevadya, with the name of Hashem.
Asking an angel for assistance: In the statement of “Zevadya..” one is requesting assistance from the angel Zevadya to guard him. This seemingly should be forbidden to be done, as one is to pray only to Hashem, and the angels have no power of their own. [See Yerushalmi Brachos 9:1 “Don’t scream to Michael or Gavriel, but to me”; Maharal Nesivos Olam Avoda 12; Shaareiy Rachamim 26 of Rav Chaim Volozhin and Maaseh Rav Sheilasos 128 [of Gr”a] who opposed the saying of Barchuni Leshalom for this reason] The explanation however is that the above prohibition is only if one Davens to the angle, however, to ask the angel to intercede on one’s behalf before G-d, is permitted. [See Sanhedrin 44b; Nefesh Chaim [Falagi] 32; Yehuda Ya’aleh 21; Chasam Sofer 166; Shemesh Utzedaka 23-24; Sefer Haminhagim p. 137 [English] and footnote 531] In any event we find many instances in Tanach, and in our prayers, in which we beseech the angels for help and blessing, with intent that they arouse mercy for us before G-d. [So said Yaakov Avinu in Hamalach Hagoel; So is said in the famous hymn of Shalom Aleichem, in the stanza of Barchuni Leshalom, in which we ask the angels for blessing; So is said in Selichos in the stanza of Machnisei Rachamim and in many other stanzas; See Yehuda Yaaleh ibid for a lengthy discussion on this subject]
 The reason: Saying the above statement of Avraham Zakein after Halel is a Segula for long life. [Machberes Hakodesh ibid; Chemdas Yamim ibid; Mishnas Chassidim ibid; Siddur Rav Shabsi ibid]
 Hayom Yom 1st Cheshvan
 Hayom Yom 1st Cheshvan
 Mishnas Chassidim ibid; Siddur Rav Shabsi ibid
 See Mishnas Chassidim ibid and all sources ibid that mention as part of the Segula to say it after Halel as a Segula for long life