Abu Dawud Book 2, Hadith Number 1370.
Narated By Abu Dharr : We fasted with the Apostle of Allah (PBUH) during Ramadan, but he did not make us get up at night for prayer at any time during the month till seven nights remained; then he made us get up for prayer till a third of the night had passed. When the sixth remaining night came, he did not make us get up for prayer. When the fifth remaining night came, he made us stand in prayer till a half of the night had gone.
So I said: Apostle of Allah, I wish you had led us in supererogatory prayers during the whole of tonight.
He said: When a man prays with an imam till he goes he is reckoned as having spent a whole night in prayer. On the fourth remaining night he did not make us get up. When the third remaining night came, he gathered his family, his wives, and the people and prayed with us till we were afraid we should miss the falah (success).
I said: What is falah? He said: The meal before daybreak. Then he did not make us get up for prayer during the remainder of the month.