Sahih Muslim Book 6, Hadith Number 2624.
Chapter : Excellence of Lailat-ul Qadar and exhortation to find it.
Abu Huraira (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) as saying: I was shown Lailat-ul-Qadr; then some members of my family awoke me up, then I was caused to forget it. So seek it in the last week. Harmala said: (The Holy Prophet did not say: “I was made to forget,” but he stated): “But I forgot it.”