Musnad Ahmad 30:2876

Ishaq narrated to us, Malik narrated to us, from Suhayl, from his father, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The gates of heaven are opened on Monday and Thursday, and every servant who does not associate anything with Allah is forgiven, except for a man between whom and his brother there is enmity. So He says: 'Delay these two until they reconcile.'"