ספר בראשית

Genesis ‎21,1. “Hashem took note of Sarah as He had ‎promised, and He did for Sarah as He had said.” Bereshit ‎Rabbah 53,4 understands this verse as reflecting the truth of ‎what the psalmist said in psalms 119,89 ‎לעולם ה', דברך נצב בשמים‎, ‎‎“The Lord exists forever; Your word stands firm in heaven.” The ‎author of the Midrash queries, rhetorically, if David meant ‎that G’d’s word does not stand firm on earth? He explains that ‎what the psalmist had in mind was that the promise G’d made to ‎Avraham He had made in heaven, i.e. when the angel announced ‎that Yitzchok’s birth would occur at a time prearranged in ‎heaven. (In Genesis 15,5, long before the angel announced ‎Yitzchok’s impending birth, G’d had take Avram outside his tent ‎and had make him look at the heaven telling him that he would ‎father children and that the would be as numerous as the stars in ‎the heaven.) For our sages in B’rachot 7 the verse is ‎understood to make the point that even when G’d makes a ‎conditional promise, He will keep it. The Talmud there uses as its ‎proof Deuteronomy 9,14 where G’d had suggested that He would ‎trade the Jewish people who had made the golden calf for a new ‎Jewish people founded by Moses.‎