Genesis 16:2-4

So Sarai said to Abram, “See here, the LORD has prevented me from bearing children. I am asking you to go in to [the bed of] my maid [so that she may bear you a child]; perhaps I will obtain children by her.” And Abram listened to Sarai and did as she said. After Abram had lived in the land of Canaan ten years, Abram’s wife Sarai took Hagar the Egyptian [maid], and gave her to her husband Abram to be his [secondary] wife. He went in to [the bed of] Hagar, and she conceived; and when she realized that she had conceived, she looked with contempt on her mistress [regarding Sarai as insignificant because of her infertility].