Halakhah zu Tehillim 3:2

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.