Al-Adab Al-Mufrad #416

Abu Hurayra reported that he heard the Prophet, may Allah bless him and grant him peace, say, "It is not lawful for a Muslim man to refuse to speak to a believer for more than three days. When three days have passed, he should meet him and greet him. If he returns the greeting, they share in the reward. If he does not return the greeting, the one who gives the greeting is innocent of having severed relations."