Halakhah su Salmi 91:1

Yehoshua b. Levi would recite two additional psalms before his sleep: “Yoshev Be-seter Elyon” (Tehilim 91) and “Hashem Ma Rabu Tzarai” (Tehilim 3), which are effective against harmful entities (Shevuot 15b); many have adopted this practice. Over the years, additional psalms and verses have been added as customary recitations. Since these are late additions, the versions differ according to community. Some recommend reciting Ve-haya im Shamo’a as well, and recommend reciting all three paragraphs.²The Sages only instituted the recitation of the first paragraph of Shema before going to sleep, as Rif (Berakhot 3a in the Rif pages), Rambam, and Rosh (Berakhot 1:6) write. However, in 9:23, Rosh cites Rabbeinu Ĥananel, who maintains that Ve-haya im Shamo’a is recited as well. Perhaps Rabbeinu Ĥananel understood the Gemara’s words “until Ve-haya im Shamo’a” to be inclusive of the second paragraph. Divrei Ĥamudot (ad loc. 7) interprets Rosh’s opinion to refer to a locale where Ma’ariv was recited before tzeit ha-kokhavim , making it necessary to recite the first two paragraphs of Shema before going to sleep. Rabbeinu Yeruĥam (3:2) and Rabbeinu Yona in Sefer Ha-yir’a state that all three paragraphs are recited, for together they contain 248 words, and saying them provides a special protection against harmful entities.

Now, we no longer have the tradition to say this ("be honored..."). This was the tradition of the first and pious whose thoughts were always devoted to G-d. About them it is said, "He who dwells in the covert of the Most High will lodge in the shadow of the Almighty" (Psalms 91:1). They felt the holiness of those who escorted them (the angels) and therefore, it was pleasant for them to say so ("be honored..."). We (in modern times), also have the escort of angles, like the sages of blessed memory have explained (and as we have explained above); however, we can not feel their holiness at all. Our saying this is considered arrogant and egotistical and it appears as arrogance as well. Certainly a holy man who is involved with Torah in holiness needs to say this, but not us, only regular people. This is the essence and this is the practice. (The "Artzot HaChaim" questions this, but the way I have written this is correct. Refer and you find it easily).

And maybe, my son, you will challenge me with that which we have the textual version in Shevuot 15b in the chapter [entitled] Yediot HaTumah, "The song of disturbances (pegaim) with harps and lyres. [...] And he says, 'He that dwells in the secret place of the Most High,' until 'Because You, O Lord, are my refuge' (Psalms 91:1-9). And he says, 'Lord, how many are my enemies become,' until 'Salvation belongs to the Lord' (Psalms 3:2-9)." And the understanding of "disturbances" is meaning to say that the saying of these psalms is beneficial in protection from damages (evil spirits). And they [further] said in Berakhot (see Rif 3a on Berakhot), "Rabbi Yehoshua ben Levi would order these verses and fall asleep." However the matter is not, God forbid, similar to the matter of invoking a charm that we mentioned. And they, may their memory be blessed, have already mentioned (Shevuot 15b), "One is prohibited from healing himself with words of Torah." Rather they mentioned to say these psalms that have things that arouse the soul of the one that knows them, to take refuge in God, may He be blessed, to put all of his trust in Him, to fix His awe in his heart and to rely upon His kindness and His goodness. And from the arousal to this, he will be protected, without a doubt, from all damage. And this is what they answered in the Gemara about this matter, as it challenged there, "And how could Rabbi Yehoshua do that? But doesn’t Rabbi Yehoshua say, 'One is prohibited from healing himself with words of Torah?'" and they said, "To protect is different" - meaning to say, the Torah did not forbid that a person say words of Torah to arouse his soul to the good, so that this merit protect him to guard him.