The Prophet (may Allah's peace & blessings be upon him, his family & companions) said: 'It is unlawful for a Muslim to avoid and refuse to speak to his fellow Muslim for more than three (days). (It is unlawful for them to behave in such a way) that when they meet, one of them turns his face away from the other, and the other turns his face from the first, The better of the two is the one who greets the other first with salaam'. (Bukhari)