Musnad Ahmad 28:285

Rawh narrated to us, Muhammad ibn Abi Hafsah narrated to us, Ibn Shihab informed us, from Sa’id ibn al-Musayyib and Abu Salamah ibn ‘Abd al-Rahman, from ‘Abdullah ibn ‘Amr ibn al-‘As, who said: It reached the Messenger of Allah (peace and blessings of Allah be upon him) that I was saying: "I will surely fast for as long as I live, and I will surely pray at night for as long as I live." So the Messenger of Allah (peace and blessings of Allah be upon him) said: "Are you the one who says, or has said: 'I will surely fast for as long as I live, and I will surely pray at night for as long as I live'?" I said: "Yes." He said: "For you will not be able to do that. So stand (to pray) and sleep, and fast and break your fast, and fast three days of every month, for the good deed is worth ten times its like." And he mentioned its meaning.