Genesis 20:17, The First Prayer Answered For Another

Then Abraham prayed to God, and God healed Abimelech, his wife and his slave girls so they could have children again.

 //The story goes that when Abraham lived in Gerar, he deceived Abimelech, king of Gerar, telling him that his wife Sarah was really his sister. So Abimelech, knowing no better, "sent for Sarah and took her." (Not really; two verses later, scripture explains that he didn't have opportunity to get near her, but it was apparently a close call. God intervened.)

Nevertheless, God is angry, and says Abimelech had better make things right with Abraham or he will die. God also "closed up every womb in Abimelech's household" so that Abimelech couldn't conceive an heir.

Abimelech pleads with Abraham, and Abraham is moved to pray to God, not only to spare Abimelech's life but to restore his ability to conceive. God hears the prayer, and opens the wombs of Abimelech's wife and slave girls.

Legend tells us that this was the first time in human history that God fulfilled the prayer of one human being for another. Not the first time someone prayed for another; Abraham had previously begged God to spare Sodom, though God refused. But Abraham's incessant pleading for others wore God down. Indeed, as the legend goes, the very reason God conversed with Abraham at all was because of Abraham's concern for others: when Abraham begged for leniency toward Sodom, God said, "You take delight in defending My creatures, and you would not call them guilty. That is why I have spoken to no one but you during the ten generations since Noah."