Sahih Muslim Book 6, Hadith Number 2634.
Chapter : Excellence of Lailat-ul Qadar and exhortation to find it.
Zirr b. Hubaish reported that Ubayy b. Ka’b (Allah be pleased with him) said about Lailat-ul-Qadr: By Allah, I know well about it. Shu’ba said: To the best of my knowledge it was the twenty-seventh night for which the Messenger of Allah (may peace be upon him) commanded us to stand for prayer. Shu’ba doubted these words: That it was the night for which the Messenger of Allah (may peace be upon him) commanded us to stand for prayer. And (he further) said: This was narrated to me by a friend of mine from him (the Holy Prophet).