Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1669
Ubayy b Ka'b reported: By Allah, I know about Lailat-ul Qadr and I know it fully well that it is the twenty-seventh night (during Ramadan) on which the Messenger of Allah (may peace be upon him) commanded us to observe prayer. (Shu'ba was in doubt about these words:" the night on which the Messenger of Allah [may peace be upon him] commanded us to observe the prayer." This has been transmitted to me by a friend of mine.)