Mishkat al-Masabih 1298

Abu Dharr said:

We fasted along with God’s Messenger, 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. Next night he did not make us get up, but when the fifth remaining night came he made us get up for prayer till a half of the night had gone, so I said, “Messenger of God, I wish you had led us in supererogatory prayers during the whole of this night." He replied, “When a man prays with an imam till he goes away he is reckoned as having spent a night in prayer." On the fourth remaining night he did not make us get up till two-thirds of the night had passed. On the third remaining night he gathered his family, his wives and the people and prayed with us till we were afraid we would miss the falah (explaining, when asked, that the falah was the meal before daybreak). Then he did not make us get up for prayer during the remainder of the month. Abu Dawud, Tirmidhi and Nasa’i transmitted it, and Ibn Majah transmitted something similar, but Tirmidhi did not mention, “then he did not make us get up for prayer during the remainder of the month."