Similarly, (Ibid. 21:1) "And the L–rd did for Sarah as He had spoken." And where did He speak it? (Ibid. 15:4) "And the 'speaking' of the L–rd was to him. This one (Ishmail) will not inherit you, etc."
God Kept His Promise to Sarah by Pointing Back to Abraham
Mekhilta Tractate Pischa 12:21