Narrated Abu Aiyub Al-Ansari: Allah's Apostle (peace be upon him)said, "It is not lawful for a man to desert his brother Muslim for more than three nights. (It is unlawful for them that) when they meet, one of them turns his face away from the other, and the other turns his face from the former, and the better of the two will be the one who greets the other first."

(Bukhari Volume 8, Book 73, Number 100)