Abu Dawud Book 2, Hadith Number 1373.
Narated By Zirr b. Hubaish : I said to Ubayy b. K’ab: Tell me about lailat al-qadr, O Abu al-Mundhir, for our companion (Ibn Mas’ud) was questioned about it, and he said: Anyone who gets up for prayer every night all the year round will hit upon it (i.e. lailat al-qadr). He replied: May Allah have mercy on Abu ‘Abd al-Rahman. By Allah he knew that it was in Ramadhan, (Musaddad’s version goes) but he disliked that the people should content themselves (with that night alone); or the people should not content themselves (with that night alone). According to the agreed version: By Allah, it is the twenty-seventh night of Ramadhan, without any reservation. I said: How do you know that, Abu al-Mundhir? He replied: By the indication (or sign) of which the Apostle of Allah (PBUH) informed us. I asked Zirr: What is that sign? He replied: The sun rises like a vessel of water in the morning following that night; it has no ray until it rises high up.