General Behavior (Kitab Al-Adab)
Dawud :: Book 41 : Hadith 4895
Narrated Aisha, Ummul Mu'minin:
The Apostle of Allah (peace_be_upon_him) said: It is not right for a Muslim to keep apart from another Muslim for more than three days. Then when he meets him and gives three salutations, receiving during that time no response, the other bears his sin.