Siman 116

The eighth blessing is "Refaeinu." And what did they see to say the blessing of healing in the eighth? Since circumcision is on the eighth day and requires healing, therefore they established it in the eighth (Megillah 17b). And it contains twenty-seven words corresponding to the twenty-seven verses in the portion of circumcision, and corresponding to the twenty-seven words in the verse "And he said, if you will diligently hearken" until "for I am Hashem your healer" (Tur). And there are twenty-seven letters in the verse "For they are life to those who find them, and healing to all his flesh" (ibid.).

The Rif, of blessed memory, at the end of the fourth chapter of Megillah brought a Tosefta, that one must not change from the wording of the verse. And according to this they raised a difficulty: for how do we say "Heal us, Hashem, and we shall be healed" in the plural? For in Jeremiah it is written in the singular: "Heal me, Hashem, and I shall be healed." And they answered that this applies to one who reads the verse or translates it, but not to those who say it by way of prayer. And when the congregation does not intend to say the verse—they may change them according to the need (Tur in the name of the Ramah). Furthermore: that this is specifically when saying the entire psalm, but when saying collected verses within the prayers, even if he intended for this verse, nevertheless since he does not say the psalm in its entirety—he is permitted to change (ibid. in the name of our teacher Yonah). And to this the Rosh, of blessed memory, agreed. Therefore, regarding the verses in Havdalah, according to the first answer it is forbidden to change them, and according to the second answer it is permitted (Taz). And so too in other matters similar to this. And one must say "Who heals the sick of His people Israel," the lamed of "cholei" with a tzeirei (see Taz).