Narrated Abu Huraira:

The Prophet peace be upon him said, "Whoever fasted the month of Ramadan out of sincere Faith (i.e. belief) and hoping for a reward from Allah, then all his past sins will be forgiven, and whoever stood for the prayers in the night of Qadr out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven."

Narrated Ibn Umar:

Some men amongst the companions of the Prophet peace be upon him were shown in their dreams that the night of Qadr was in the last seven nights of Ramadan. Allah's Messenger peace be upon him said, "It seems that all your dreams agree that (the Night of Qadr) is in the last seven nights, and whoever wants to search for it (i.e. the Night of Qadr) should search in the last seven (nights of Ramadan).''