Abu Ayyub Ansiri reported Allah's Messenger (sallallaahu alayhi wa sallam) as saying:
It is not permissible for a Muslim to have estranged relations with his brother beyond three nights, the one turning one way and the other turning the other way when they meet; the better of the two is one who is the first to give a greeting.
Sahiih Muslim Hadiith No. 2560
Abu Hurairah (May Allah be pleased with him) said:
The Messenger of Allah (sallallaahu alayhi wa sallam) said, "It is not lawful for a Muslim to forsake his (Muslim) brother beyond three days; and whosoever does so for more than three days, and then dies, will certainly enter the Hell."
Sunan Abi Daawud Hadiith No. 1595
أفاد الحديث : التغليظ والتشديد في أمر الهجر والخصام ، فمن هجر أخاه فوق ثلاث ليال ، ولم يتب إلى الله : فقد استوجب العقوبة بدخول النار ، إلا أنه في مشيئة الله : إن شاء عذبه ، وإن شاء غفر له
This Hadiith is a severe warning regarding disputes and forsaking your Muslim brother for over three nights. If a person forsakes his Muslim brother for more than three nights and does not repent to Allaah, then he will be punished by being thrown into the Fire unless Allaah Wills otherwise. Allaah may punish him if He Wills, and He may forgive him if He Wills.
إلا من استحل ذلك ، مع علمه بحرمته ، ولم يحرم حرام الله في ذلك ، ولم يلتزمه : فإن هذا ردة عن دين الله عز وجل
The exception is the one who considers this halaal knowing that it is haraam and does not make haraam what Allaah has made haraam in regards to that. This is apostasy from the Diin of Allaah.