"Then Leah bore me, and I was called Issachar, on account of the reward Rachel had given to my mother. At that time an angel of the Lord appeared to Jacob, and he spoke: 'Rachel will bear only two sons, for she rejected the espousal of her husband, and chose continence! But Leah bore six sons, for the Lord knew that she desired to be with her husband, not because she was prompted by the evil inclination, but for the sake of children. Rachel's prayer also was fulfilled, on account of the dudaim, for although she desired to eat of the apples, she did not touch them, but put them in the house of the Lord, and gave them to the priest of the Most High that was in those days.
The Mandrakes That Caused a Bargain Between Rachel and Leah
Legends of the Jews 2:40