Musannaf Ibn Abi Shayba 9:1016

Ubayd Allah narrated to us, from Isra'il, from al-Sari, from Abdullah ibn Shaddad, regarding His saying: "Nor to exchange them for other wives," he said: That means if he were to divorce them, it would not be lawful for him to exchange them. The Prophet, may Allah bless him and grant him peace, used to marry whomever he wished after it was revealed, and it was revealed while he had nine wives under him, and he married Umm Habibah and Juwayriyah.