Abu Hurairah (May Allah be pleased with him) said:
The Messenger of Allah (ﷺ) said, "It is not permissible for a believer to forsake his (Muslim) brother for more than three days. If three days have passed, he should meet him and greet him; and if other responds to it they will both share the reward; but if he does not respond, he will bear his sin and the one who (has taken the initiative to) greet (the other) will be absolved of the sin of forsaking (one's brother in Faith)."[Abu Dawud].
App reference: Book 18, Hadith 87
Arabic/English book reference: Book 18, Hadith 1597