Musnad Ahmad 28:166

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to us, from Abu Abd al-Rahman al-Hubuli, from Abdullah ibn Amr, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Allah, the Almighty and Majestic, looks upon His creation on the night of the middle of Sha'ban, and He forgives His servants, except for two: one who harbors hatred, and one who kills a soul."