Musnad Ahmad 298:8

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from Yazid al-Rishk, from Mu‘adhah, from Hisham ibn ‘Amir, that he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "It is not lawful for a Muslim to forsake a Muslim for more than three nights, for they are both turning away from the truth as long as they remain in their estrangement. The first of them to return [to reconciliation] will have his return as an expiation for him. If he greets him and he does not accept it and rejects his greeting, the angels will return the greeting for him, and the devil will return it for the other. If they die in their state of estrangement, they shall never enter Paradise together."