صحیح مسلم · Hadith 2778

Zirr b. Habaish reported: I thu asked Ubayy b. Ka'b (Allah be pleased with him): Your brother (in faith) Ibn Mas'ud says: He who stands (for the night prayer) throughout the year would find Lailat-ul-Qadr, whereupon he said: May Allah have mercy upon him; (he said these words) with the intention that people might not rely only (on one night), whereas he knew that it (Lailat-ul-Qadr) is in the month of Ramadan and it is the twenty-seventh night. He then took oath (without making any exception, i. e. without saying In sha Allah) that it was the twenty-seventh night. I said to him: Abu Mundhir, on what ground do you say that? Thereupon he said: By the indication or by the sign which the Messenger of Allah ( ‌صلی ‌اللہ ‌علیہ ‌وسلم ‌ ) gave us, and that is that on that day (the sun) would rise without having any ray in it.