Genesis 30:1-5

When Rachel saw that she conceived no children for Jacob, she envied her sister, and said to Jacob, “Give me children, or else I will die.” Then Jacob became furious with Rachel, and he said, “Am I in the place of God, who has denied you children?” She said, “Here, take my maid Bilhah and go in to her; and [when the baby comes] she shall deliver it [while sitting] on my knees, so that by her I may also have children [to count as my own].” So she gave him Bilhah her maid as a [secondary] wife, and Jacob went in to her. Bilhah conceived and gave birth to a son for Jacob.