Vayetze 

ויצא

While Leah was blessed with many children, Rachel remained barren. She demanded from Jacob: “Give me children, and if not, I am dead” (Genesis 30:1). The Sages describe the dialogue between them and Jacob’s suggested solution.  

Jacob responded to Rachel: “Am I in place of God who withheld from you fruit of the womb? (30:2). He withheld children from you; He did not withhold children from me [as I have several children from Leah]. Rachel said to him: Is that what your father, Isaac, did to your mother? Didn’t he gird his loins [i.e., pray vigorously] across from her? Jacob said to her: My father did not have children, but I have children. Rachel said to him: But didn’t your grandfather, Abraham, already have children from Hagar and he girded his loins across from Sarah? Jacob said to her: You can do what my grandmother Sarah did. Rachel said to him: What did she do? Jacob said to her: She introduced her rival into her household by having Abraham marry Hagar. Rachel said to him: If that is the hindrance to my conceiving, I agree. “She said: Here is my maid Bilha, consort with her…and I will be built through her” (30:3). Just as this one, Sarah, was built through her rival, so too, that one, I, will be built through my rival. <source>(<i>Bereshit</i> <i>Rabba</i> 71)  </source>