Hadith: It is unlawful for a Muslim to desert his fellow Muslim for more than three nights. As they meet, both of them turn their backs on each other. But the best of them is the one who starts by greeting the other

عن أبي أيوب الأنصاري رضي الله عنه أن رسول الله صلى الله عليه وسلم قال:
«لَا يَحِلُّ لِرَجُلٍ أَنْ يَهْجُرَ أَخَاهُ فَوْقَ ثَلَاثِ لَيَالٍ، يَلْتَقِيَانِ، فَيُعْرِضُ هَذَا وَيُعْرِضُ هَذَا، وَخَيْرُهُمَا الَّذِي يَبْدَأُ بِالسَّلَامِ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 6077]

Abu Ayyūb al-Ansāri (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) said:
"It is unlawful for a Muslim to desert his fellow Muslim for more than three nights. As they meet, both of them turn their backs on each other. But the best of them is the one who starts by greeting the other."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 6077]

Explanation

The Prophet (may Allah's peace and blessings be upon him) forbade that a Muslim should desert his fellow Muslim for more than three nights. None of them greets the other or talks to him when they meet.
The best of such two estranged persons is the one who tries to end their estrangement, so he greets the other first. The kind of desertion intended in this Hadīth is what a person engages in for his own sake. However, a person may desert others for the sake of Allah Almighty, like deserting sinners, heretic innovators, and bad companions. This does not have a time limit. Instead, it is contingent on a cause and should end when the cause is no longer there.