Musannaf Ibn Abi Shayba 10:273

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Mansur, from Ibrahim, and from Jabir, from ‘Amir, and from ‘Imran ibn Muslim, from Ibn ‘Aqal, from Mus’ab ibn Sa’d, Abu Malik, and ‘Abdullah ibn Shaddad; they said: If a man divorces his wife three times while she is pregnant, she is not lawful for him until she marries a husband other than him.