Sahih Al-Bukhari
(The Book of Asking Permission)
:: Vol 8 :: Book 74 :: Hadith 254 ::
Narrated Abu Aiyub (R.A) :
The Prophet (S.A.W) said, It is not lawful for a Muslim to desert (not to speak to) his brother Muslim for more than three days while meeting, one turns his face to one side and the other turns his face to the other side. Lo! The better of the two is the one who starts greeting the other.