Sahih Bukhari
Asking Permission
Volume: 008 / Book Number: 074 / Hadith Number: 254
Narrated Abu Aiyub:
The Prophet 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."