Musnad Abu Ya'la al-Mawsili 78:6

Hisham bin Amir al-Ansari said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "It is not lawful for a Muslim to shun a Muslim for more than three [days]. They are both turning away from the truth as long as they are in their state of shunning. The first of them to return [to reconciliation] will have his initiative to return as an expiation for him. If he greets him and he does not accept his greeting, the angels will return [the greeting] to him, and the devil will return [the greeting] to the other. If they die while in their state of shunning, they will not enter Paradise, or they will not be gathered together in Paradise."