Mishkat al-Masabih 317 — Purification

‘Ali reported God's messenger as saying, “The leather strap of the anus is the eyes, so one who sleeps should perform ablution. Abu Dawud transmitted it, and the shaikh and imam Muhyi as-Sunna said that this does not refer to one who is sitting because of the sound tradition from Anas who said that the Companions of God’s messenger dozed while waiting for nightfall, then prayed without performing ablution. Abu Dawud and Tirmidhi transmitted it, but Tirmidhi said “they slept” instead of saying that they “dozed while waiting for nightfall.”