Al-Adab Al-Mufrad, Book 22, Hadith 399 (English) ⓘ

Abu Ayyub, the Companion of the Messenger of Allah, may Allah bless him and grant him peace, said, "It is not lawful for anyone to cut himself off from his Muslim brother for more than three nights so that when they meet, one of them turns his face away in avoidance and the other one turns his face away as well. The better of them is the one who initiates the greeting."

حَدَّثَنَا عَبْدُ اللهِ بْنُ صَالِحٍ قَالَ‏:‏ حَدَّثَنِي اللَّيْثُ قَالَ‏:‏ حَدَّثَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، عَنْ عَطَاءِ بْنِ يَزِيدَ اللَّيْثِيِّ ثُمَّ الْجُنْدَعِيِّ، أَنَّ أَبَا أَيُّوبَ صَاحِبَ رَسُولِ اللهِ صلى الله عليه وسلم أَخْبَرَهُ أَنَّ رَسُولَ اللهِ صلى الله عليه وسلم قَالَ‏:‏ لاَ يَحِلُّ لأَحَدٍ أَنْ يَهْجُرَ أَخَاهُ فَوْقَ ثَلاَثِ لَيَالٍ، يَلْتَقِيَانِ فَيَصُدُّ هَذَا وَيَصُدُّ هَذَا، وَخَيْرُهُمَا الَّذِي يَبْدَأُ بِالسَّلامِ‏.‏