Muwatta Malik #248

Yahya related to me from Malik from Ibn Shihab from Urwa ibn az- Zubayr from A'isha, the wife of the Prophet, may Allah bless him and grant him peace, that the Messenger of Allah, may Allah bless him and grant him peace, prayed in the mosque one night and people prayed behind him. Then he prayed the next night and there were more people. Then they gathered on the third or fourth night and the Messenger of Allah, may Allah bless him and grant him peace, did not come out to them. In the morning, he said, "I saw what you were doing and the only thing that prevented me from coming out to you was that I feared that it would become obligatory (fard) for you." This happened in Ramadan.