Musannaf Ibn Abi Shayba 21:739

Abu Khalid al-Ahmar narrated to us, from Hajjaj, from Makhul, from Kathir ibn Murrah al-Hadrami, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah descends on the night of the middle of Sha'ban, and He forgives sins therein, except for a polytheist or a person harboring enmity."