Genesis 30:3-8

Then she said, “Here is Bilhah, my servant. Sleep with her so that she can bear children for me and I too can build a family through her.” So she gave him her servant Bilhah as a wife. Jacob slept with her, and she became pregnant and bore him a son. Then Rachel said, “God has vindicated me; he has listened to my plea and given me a son.” Because of this she named him Dan. Rachel’s servant Bilhah conceived again and bore Jacob a second son. Then Rachel said, “I have had a great struggle with my sister, and I have won.” So she named him Naphtali.