Al-Adab Al-Mufrad, Book 22, Hadith 402 (English) ⓘ

Hisham ibn 'Amir al-Ansari, the cousin of Anas ibn Malik whose father was killed in the Battle of Uhud, that he 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 each other, neither of them will ever enter the Garden. If one of them greets the other and he refuses to return the greeting or accept his greeting, then an angel returns the greeting to him and Shaytan answers the other."

حَدَّثَنَا أَبُو مَعْمَرٍ، قَالَ‏:‏ حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنْ يَزِيدَ قَالَ‏:‏ قَالَتْ مُعَاذَةَ‏:‏ سَمِعْتُ هِشَامَ بْنَ عَامِرٍ الأَنْصَارِيَّ، ابْنَ عَمِّ أَنَسِ بْنِ مَالِكٍ، وَكَانَ قُتِلَ أَبُوهُ يَوْمَ أُحُدٍ، أَنَّهُ سَمِعَ رَسُولَ اللهِ صلى الله عليه وسلم قَالَ‏:‏ لاَ يَحِلُّ لِمُسْلِمٍ أَنْ يُصَارِمَ مُسْلِمًا فَوْقَ ثَلاَثٍ، فَإِنَّهُمَا نَاكِبَانِ عَنِ الْحَقِّ مَا دَامَا عَلَى صِرَامِهِمَا، وَإِنَّ أَوَّلَهُمَا فَيْئًا يَكُونُ كَفَّارَةً عَنْهُ سَبْقُهُ بِالْفَيْءِ، وَإِنْ مَاتَا عَلَى صِرَامِهِمَا لَمْ يَدْخُلاَ الْجَنَّةَ جَمِيعًا أَبَدًا، وَإِنْ سَلَّمَ عَلَيْهِ فَأَبَى أَنْ يَقْبَلَ تَسْلِيمَهُ وَسَلاَمَهُ، رَدَّ عَلَيْهِ الْمَلَكُ، وَرَدَّ عَلَى الْآخَرِ الشَّيْطَانُ‏.‏