Musnad Ahmad 1156:235

Hajjaj narrated to us, from Layth ibn Sa'd, who said: Bukayr narrated to me, from Abu Bakr ibn al-Munkadir, from Abu Salamah ibn 'Abd al-Rahman, from Zaynab bint Abi Salamah, from Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, who said: "The Messenger of Allah, may Allah bless him and grant him peace, kissed [her] while he was fasting."