Musnad Ahmad 27:1839

Ibn Numayr narrated to us, Ubayd Allah narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) fasted continuously (wisal) in Ramadan. The people saw him doing so, so he forbade them. It was said to him: "You yourself fast continuously." He said: "I am not like you; I am fed and given to drink."