Sahih Muslim Book 4, Hadith Number 1668.

Chapter : Exhortation to supplicate and make mention (of Allah) at the end of the night and its acceptance by the Lord.

Zirr (b. Hubaish) reported: I heard from Ubayy b. Ka’b a statement made by ‘Abdullah b. Mas’ud in which he said: He who gets up for prayer (every night) during the year will hit upon Lailat-ul-Qadr. Ubayy said: By Allah I there is no god but He, that (Lailat-ul-Qadr) is in Ramadhan (He swore without reservation:) By Allah, I know the night; it is the night on which the Messenger of Allah (may peace be upon him) commanded us to pray. It is that which precedes the morning of twenty-seventy and its indication is that the sun rises bright on that day without rays.