Musnad Ahmad 937:130

Ali ibn Asim narrated to us, from Dawud, from al-Walid ibn Abd al-Rahman, from Jubayr ibn Nufayr, from Abu Dharr, who said: We fasted Ramadan with the Messenger of Allah (peace and blessings of Allah be upon him), and he did not stand with us [in prayer] for any part of the month, until it was the twenty-fourth night. The Messenger of Allah (peace and blessings of Allah be upon him) stood with us until it was almost a third of the night. When the following night came, he did not stand with us. When it was the twenty-sixth night, the Messenger of Allah (peace and blessings of Allah be upon him) stood with us until it was almost half of the night. I said: "O Messenger of Allah, if only you would grant us the rest of this night of ours [in prayer]." He said: "No, for indeed, when a man stands with the Imam until he departs, it is recorded for him as the standing of a [full] night." When the following night came, he did not stand with us. When it was the twenty-eighth night, the Messenger of Allah (peace and blessings of Allah be upon him) gathered his family, and the people gathered for him, and the Messenger of Allah (peace and blessings of Allah be upon him) prayed with us until we almost missed the *falah*. I said: "And what is the *falah*?" He said: "The *suhur* (pre-dawn meal)." Then he did not stand with us, O son of my brother, for any [remaining] part of the month.