Musnad Ahmad 298:7

Rawh ibn ‘Ubadah narrated to us, he said: Shu‘bah narrated to us, from Yazid al-Rishk—Shu‘bah said: I read it to him—he said: I heard Mu‘adhah al-‘Adawiyyah say: I heard Hisham ibn ‘Amir say: I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "It is not lawful for a Muslim to forsake a Muslim for more than three nights. If they remain estranged for more than three, they are both deviating from the truth as long as they remain in their estrangement. The first of them to return [to reconciliation] and precede the other with the return, it will be an expiation for him. If he greets him and he does not return his greeting, and he returns his greeting, the angels return [the greeting] to him, and the devil returns [the greeting] to the other. If they die in their state of estrangement, they shall never be gathered together in Paradise."