Musnad Ahmad 32:1069

Bahz narrated to us, and Hajjaj narrated to us, they said: Sulayman ibn al-Mughirah narrated to us, the meaning, from Thabit, from Anas, who said: The Prophet, peace and blessings of Allah be upon him, was praying during Ramadan, so I came and stood behind him. He said: And a man came and stood by my side, then another came until we were a group. When the Messenger of Allah, peace and blessings of Allah be upon him, sensed that I was behind him, he shortened the prayer. Then he stood up and entered his house, and he prayed a prayer he had not prayed with us. He said: When morning came, we said: "O Messenger of Allah, did you notice us last night?" He said: "Yes, and that is what compelled me to do what I did." He said: Then he began to fast continuously (wisal), and that was at the end of the month. He said: So some men among his companions began to fast continuously. He said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "What is the matter with men who fast continuously? You are not like me. By Allah, if the month were extended for me, I would have fasted continuously in a way that would make those who go to extremes abandon their extremism."