And the Lord remembered Sarah as he had said, and the Lord did unto Sarah a miracle as Abraham had spoken in his prayer on behalf of Abimelech.

And Sarah conceived, and bare Abraham a son like to him in his age, at the set time of which the Lord had spoken to him.

Notes on Genesis xxi. 1–2

Raba said to Rabba b. Mari: ‘On what do we base the rabbinic deduction that a man who asks consideration for another when he is in need of the same thing himself is heard as well?’ He said, ‘It is written, And the Lord turned the captivity of Job, when he prayed for his friends’. He said, ‘You quote that passage, but I say it is, And Abraham prayed unto God: and God healed Abimelechy and his wife and his maidservants, and later goes on, And the Lord visited Sarah as he had said, that is, as Abraham had said in the case of Abimelech’.

Tanḥ.B. i. 106 commented: ‘The ministering angels said to the Holy One, blessed be he, “Lord of the universe, Abraham heals others but himself needs healing. He has healed Abimelech and his house, and they have borne children,…but you do not heal him.” The Holy One, blessed be he, said, “He is worthy that I should give him sons”.’