Musnad Ahmad 1153:2000

Yazid ibn Harun narrated to us, he said: Al-Hajjaj ibn Artah informed us, from Yahya ibn Abi Kathir, from Urwah, from Aisha, who said: I missed the Messenger of Allah, peace and blessings of Allah be upon him, one night, so I went out, and behold, he was at Al-Baqi, raising his head toward the sky. He said to me: "Did you fear that Allah and His Messenger would deal unjustly with you?" She said: I said, "O Messenger of Allah, I thought you had gone to one of your wives." He said: "Indeed, Allah, the Almighty and Majestic, descends on the night of the middle of Sha'ban to the lowest heaven, and He forgives more than the number of hairs on the sheep of [the tribe of] Kalb."