Sahih Bukhari Volume 8, Book 73, Hadith Number 99.

Narated By Anas bin Malik : Allah’s Apostle said, “Do not hate one another, nor be jealous of one another; and do not desert one another, but O Allah’s worshipers! Be Brothers! And it is unlawful for a Muslim to desert his brother Muslim (and not to talk to him) for more than three nights.”