Sahih Bukhari
Good Manners and Form (Al-Adab)
Volume: 008 / Book Number: 073 / Hadith Number: 91
Narrated Anas bin Malik:
Allah's Apostle said, "Do not hate one another, and do not be jealous of one another, and do not desert each other, and O, Allah's worshipers! Be brothers. Lo! It is not permissible for any Muslim to desert (not talk to) his brother (Muslim) for more than three days."