Rashi on Genesis 21:2/רש"י על בראשית כ״א:ב׳

1 למועד אשר דבר אתו Render it as Onkelos translates: “At the appointed time which He had spoken: — the time He had said and appointed. When He had said to him (18:4) “At the set time I will return unto thee”, He scratched a mark in the wall and said to him (Genesis 53:6), “When the sunrays touch this mark next year she will bear a son” (Yalkut Shimoni on Torah 59:1).