Musannaf Ibn Abi Shayba 9:862

Ibn Fudayl narrated to us, from al-A’mash, from Ibrahim, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "A woman shall not be married [as a co-wife] alongside her paternal aunt, nor alongside her maternal aunt. And a man shall not have intercourse with a woman while there is a fetus in her womb from someone else."