Al-Adab Al-Mufrad, Book 22, Hadith 406 (English) ⓘ

Abu Ayyub al-Ansari reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "It is not lawful for a Muslim to refuse to speak to his (Muslim) brother for more than three days so that when they meet, one goes this way and the goes that way. The better of the two is the one who initiates the greeting."

حَدَّثَنَا إِسْمَاعِيلُ قَالَ‏:‏ حَدَّثَنِي مَالِكٌ، عَنِ ابْنِ شِهَابٍ، عَنْ عَطَاءِ بْنِ يَزِيدَ اللَّيْثِيِّ، عَنْ أَبِي أَيُّوبَ الأَنْصَارِيِّ، أَنَّ رَسُولَ اللهِ صلى الله عليه وسلم قَالَ‏:‏ لاَ يَحِلُّ لِمُسْلِمٍ أَنْ يَهْجُرَ أَخَاهُ فَوْقَ ثَلاَثَةِ أَيَّامٍ، يَلْتَقِيَانِ فَيُعْرِضُ هَذَا وَيُعْرِضُ هَذَا، وَخَيْرُهُمَا الَّذِي يَبْدَأُ بِالسَّلامِ‏.‏