Sahih Bukhari
Good Manners and Form (Al-Adab)
Volume: 008 / Book Number: 073 / Hadith Number: 99
Narrated 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."