Al-Adab Al-Mufrad #409

Hisham ibn 'Amir heard the Messenger of Allah, may Allah bless him and grant him peace, say, "It is not lawful for a Muslim to snub another Muslim for more than three nights. As long as they are cut off from each other, they are turning away from the Truth. The first of them to return to a proper state has his expiation for that inasmuch as he was the first to return to a proper state. If they die while they are cut off from one another, neither of them will ever enter the Garden."