Musnad Abu Ya'la al-Mawsili 149:88

Abu Bakr narrated to us, Ibn Numayr narrated to us, from Ibn Ishaq, from Thawr, from Muhammad ibn Ubayd ibn Abi Salih, from Safiyyah bint Shaybah, from Aisha, that the Messenger of Allah (may Allah bless him and grant him peace) said: "There is no divorce and no manumission in a state of coercion (ighlaq)."