Good Manners and Form (Al-Adab)
Ṣaḥīḥ al-Bukhārī 6077
لاَ يَحِلُّ لِرَجُلٍ أَنْ يَهْجُرَ أَخَاهُ فَوْقَ ثَلاَثِ لَيَالٍ، يَلْتَقِيَانِ فَيُعْرِضُ هَذَا وَيُعْرِضُ هَذَا، وَخَيْرُهُمَا الَّذِي يَبْدَأُ بِالسَّلاَمِ
lā yaḥillu lirajulin an yahjura akhāhu fawqa thalāthi layālin, yaltaqiyāni fayuʿriḍu hadhā wayuʿriḍu hadhā, wakhayruhumā adh-dhī yabdaʾu bi-as-salāmi
Transliteration generated from the Arabic vowel marks
The full narration
Narrated Abu Aiyub Al-Ansari:Allah's Messenger (ﷺ) said, "It is not lawful for a man to desert his brother Muslim for more than three nights. (It is unlawful for them that) when they meet, one of them turns his face away from the other, and the other turns his face from the former, and the better of the two will be the one who greets the other first
Source
Ṣaḥīḥ al-Bukhārī · 6077