אורח חיים קט״זOrach Chayim 116

Se’if 1א׳

רְפָאֵנוּ ה' וְנֵרָפֵא, אַף עַל גַּב דְּהַכָּתוּב לְיָחִיד אֵין מְכַנִּין אוֹתוֹ לְרַבִּים, הָנֵי מִלֵּי בַּזְּמַן שֶׁמִּתְכַּוֵּן לִקְרוֹת, אֲבָל כְּשֶׁאוֹמֵר אוֹתוֹ דֶּרֶךְ תְּפִלָּה וּבַקָּשָׁה, מֻתָּר. וּמִכָּל מָקוֹם אִם אוֹמֵר מִזְמוֹר שָׁלֵם, אָסוּר לְשַׁנּוֹת מִלְּשׁוֹן יָחִיד לְרַבִּים, אוֹ לְהֶפֶךְ (טוּר וְהָרֹא"שׁ פֶּרֶק הַקוֹרֵא עוֹמֵד בְּשֵׁם הָר"י).

Commentary on the the Blessing "Heal us". Containing 1 Se'if: "Heal us, O God, and we shall be healed..." Even though a verse that is written in the singular may not be modified to the plural [i.e. like the beginning words of this blessing which in Jeremiah 17:14 is written in the singular "heal me"] - this applies in the case when it was intended to be recited [as a verse], but when one says it in the context of a prayer or a request, it is permitted [to modify it]. Gloss: Nevertheless, if one says an entire psalm, it is forbidden to alter it from singular to plural or vice versa. (Tur and the Rosh on the chapter "Hakoreh Omed" [Talmud Megillah 4] in the name of the Ri]