Jami` at-Tirmidhi Hadith 794

"My father narrated to me: 'The Night of Al-Qadr was mentioned in the presence of Abu Bakrah, so he said: "I do not search for it due to something that I heard from the Messenger of Allah, except for during the last ten (nights); for indeed I heard him say: 'Search for it when nine remain, or; when seven remain, or; when five remain, or; during the last three nights.'" He (Uyainah) said: "During the twenty (nights) of Ramadan, Abu Bakrah used to perform Salat just as he performed Salat during the rest of the year. But when the (last) ten began, he would struggle (performing more Salat during the night)."