Yahya related to me from Malik from Ibn Shihab from Ata ibn Yazidal-Laythi from Abu Ayyub al-Ansari that the Messenger of Allah, mayAllah bless him and grant him peace, said, "It is not halal for amuslim to shun his brother for more than three nights, that is theymeet, and this one turns away and that one turns away. The better ofthe two is the one who says the greeting first."
USC-MSA web (English) reference: Book 47, Hadith 13
Arabic reference: Book 47, Hadith 1648