Muhammad bin Hilal narrates from his father: I heard Sayyiduna Abu Hurayrah (may Allah be pleased with him) say that the Prophet (peace and blessings be upon him) said: "It is not lawful for a believer to forsake his believing brother for more than three days; but after three days have passed, they should greet each other with salam. If he replies, both of them will share in the reward; and if he does not reply, then this one (who did not reply) will be the wrongdoer, and the other will be free of (the sin of) cutting off relations."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20028