Musnad Ahmad Hadith 21193

Zirr says that I asked Ubayy bin Ka'b (may Allah be pleased with him) that your brother Abdullah bin Mas'ud (may Allah be pleased with him) says that whoever stands (in prayer) the whole year can attain Laylat al-Qadr? Ubayy (may Allah be pleased with him) replied: May Allah's mercy descend upon him, he knows that Laylat al-Qadr is in the month of Ramadan and it is on the twenty-seventh night (but he wants to keep it hidden from people so that they do not rely solely on it). Then he swore an oath (by the One who revealed the Book to Muhammad, peace be upon him, Laylat al-Qadr is on the twenty-seventh night of Ramadan). I asked, (O Abu al-Mundhir!) how do you know this? He replied: By the sign that has been told to us, that on the morning after this night, when the sun rises, it has no rays.