Abu Dawud Book 36, Hadith Number 4894.
Chapter : Not known.
Narated By AbuHurayrah : The Prophet (PBUH) said: It is not allowable for a believer to keep from a believer for more than three days. If three days pass, he should meet him and give him a salutation, and if he replies to it they will both have shared in the reward; but if he does not reply he will bear his sin (according to Ahmad’s version) and the one who gives the salutation will have come forth from the sin of keeping apart.