Musannaf Ibn Abi Shayba 9:312

Mu'adh ibn Mu'adh narrated to us, from Ash'ath, from al-Hasan and Muhammad, they both said: "Three verses are ambiguous: 'And the wives of your sons,' and 'what your fathers married,' and 'and the mothers of your wives.'" Ash'ath said: "And this fourth one is not ambiguous: 'And your stepdaughters who are under your guardianship, born of your wives unto whom you have gone in.'" Then Mu'adh recited it to its end.