Genesis 16:1 - 16
NKJV - 1 Now Sarai, Abram's wife, had borne him no children. And she had an Egyptian maidservant whose name was Hagar. 2 So Sarai said to Abram, "See now, the LORD has restrained me from bearing children. Please, go in to my maid; perhaps I shall obtain children by her." And Abram heeded the voice of Sarai.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.