לֹא־אֲחַלֵּ֥ל בְּרִיתִ֑י וּמוֹצָ֥א שְׂ֝פָתַ֗י לֹ֣א אֲשַׁנֶּֽה׃
내 언약을 파하지 아니하며 내 입술에서 낸 것도 변치 아니하리로다
Rav Hirsch
מוצא פי ה׳ (vergl. Jes. 55, 11). כן יהיה דברי אשר יצא מפי, wo damit jede von Gott ausgehende Bestimmung bezeichnet wird. So heißt auch beim Menschen das Wort, das, sei es als Gelübde, oder als Wunsch, Tat werden soll: מוצא שפתיו. Dewarim 23, 24 und Jirmija 17, 16). So auch von Gott die über die Davidische Dynastie ausgesprochene Bestimmung: ומוצא שפתי לא אשנה (Ps. 89, 35).
Rashi
כי עתה ידעתי FOR NOW I KNOW—R. Aba said: Abraham said to God, “I will lay my complaint before you. Yesterday (on an earlier occasion) you told me, (Genesis 21:12) “In Isaac shall seed be called to thee”, and then again you said, (Genesis 21:2) “Take now thy son”. Now you tell me, “Lay not thy hand upon the lad”! The Holy One, blessed be He, said to him, in the words of Psalms 89:35, “My covenant will I not profane, nor alter that which is gone out of My lips”. When I told you, “Take thy son”, I was not altering that which went out from My lips, namely, My promise that you would have descendants through Isaac. I did not tell you “Slay him” but bring him up to the mountain. You have brought him up — take him down again” (Genesis Rabbah 56:8).