Musnad Ahmad 1153:1875

'Abd al-Razzaq narrated to us, he said: Ma'mar narrated to us, from al-Zuhri, from 'Urwah, who said: The wife of 'Uthman ibn Maz'un—I think her name was Khawlah bint Hakim—entered upon 'Aishah while she was in a disheveled state. She asked her, "What is the matter with you?" She said, "My husband stands in prayer at night and fasts during the day." Then the Prophet, peace and blessings of Allah be upon him, entered, and 'Aishah mentioned that to him. The Messenger of Allah, peace and blessings of Allah be upon him, met 'Uthman and said, "O 'Uthman, monasticism has not been prescribed for us. Do you not have an example in me? For by Allah, I am the most fearful of Allah among you, and the most observant of His limits among you."