Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.145A.

Scholars hold different opinions as to the night which is the night of qadr. Some are of the opinion that it is the 21st, some say the 23rd, others say the 25th and still others say it is the 29th. Some say that it varies from year to year but it is always among the last ten nights of Ramadan. Most scholars, though, vouch for the 27th.

Ahmad recorded, with a sahih chain, from Ibn ‘Umar that the Prophet said: “He who likes to seek that night should do so on the 27th. Ubayy ibn K’ab said: By Allah, and there is no God but Him, it is during Ramadan–and He swore to that–and by Allah, I know what night it is. It is the night during which the Prophet ordered us to make prayers, the night of the 27th. Its sign is that the sun rises in the morning white and without any rays.” This is related by Muslim, Abu Dawud, Ahmad, and by at-Tirmizhi who called it sahih.