Saying the names of the angels:[1]

The Arizal was very careful to not verbalize the names of angels and considered it a prohibition. He abstained from mentioning even those names of angels which are written in Sefarim [and are known to all]. He was particular in this even when he taught Torah to students, and whenever he needed to mention the name of an angel, he would say it in abbreviation. The Arizal states that one who is not careful in this and says the name in full causes the angel to take vengeance against him.[2] Accordingly, the widespread custom of Jewry is to beware not to verbalize the names of angels with exception to those angels who have a similar name to men.[3]

Matatron and Samael: One should not say the name Matatron but rather should simply say “Mem Tes.” Likewise, one should not say the name Samael, but rather one should say Samech Mem.[4] One who mentions the name of the angel called Samael, transgresses the prohibition of “Visheim Elokim Acheirim Lo Sazkiru.”[5]

Angels with common names of people: One may say the name Michael and Gavriel and Rafael being that these are also names of people.[6] Likewise, due to this reason one may say the name Zevadya.[7] [Likewise, due to this reason one may say the name Uriel and Muriel.[8]]

Mentioning names in prayer:[9] The above prohibition against mentioning the names of angels applies even during prayer, and a Yehi Ratzon supplication.

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[1] Custom of Arizal, brought in Shaar Hamitzvos Shemos p. 26; Nagid Mitzvah in Seder Hakeria, Shaar Hakavanos Drush Alef Tefilas Arvis; Shelah p. 100; Kav Hayashar 53; Zechor Leavraham 3 Mareches Habayis; Kitzur SHU”A 129:15; Torah Leshma 426; Kaf Hachaim 5:10; Taamei Haminhagim Likkutim 18 in name of Kitzur Shelah; Sefer Segulas Yisrael Mareches 1:69; Sefer Shemiras Haguf Vihanefesh [Lerner] 90:3 footnote 3

[2] The reason: As angels are considered servants of G-d, and G-d arranged that as soon as their name is mentioned in a swear, that the angel arrive. Due to this, as soon as their name is mentioned they appear and take vengeance against those who call them [unjustifiably] and instigate them and prosecute against them. [Shaar Hamitzvos ibid]

[3] Torah Lishma ibid

[4] Shaar Hamitzvos Shemos p. 26

[5] Shaar Hamitzvos Parshas Mishpatim; Kaf Hachaim ibid

[6] Shaar Hamitzvos Shemos p. 26; Torah Lishma ibid

[7] Sefer Segulas Yisrael ibid; Shaar Hakolel 37:10; See Nefesh Chaim [Falagi] 32

See regarding the statement of Zevadya Yishmireini, said on Rosh Chodesh after Hallel: 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 old manuscript; Kaf Hachaim 422:57; It is omitted in Siddur Harizal and in Peri Eitz Chaim [Shaar Hakolel ibid] 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. 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.

[8] Shemiras Haguf Vihanefesh ibid

[9] Kitzur SHU”A 129:15