Musnad Ahmad

Zirr says that once I visited Madinah Munawwarah and entered Masjid Nabawi, where I met Hazrat Ubayy bin Ka'b (may Allah be pleased with him). I said, "O Abu al-Mundhir! May Allah's mercy descend upon you, show kindness to me." In fact, there was a little sternness in his temperament. I asked him about Laylat al-Qadr, so he declared it to be the twenty-seventh night. I said, "O Abu al-Mundhir! How do you know this?" He replied, "By the sign that has been told to us: on the morning following that night, when the sun rises, it has no rays."