Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "It is unlawful for a Muslim to abandon his brother for more than three (days); so whoever abandons more than three then dies will enter the Hellfire."
Meaning of the Hadīth: It is not lawful for the Muslim to abandon his Muslim brother more than three days, if the abandonment is for some personal reason or for some worldly affair. However, if it is done for a valid religious purpose, it will be permissible or perhaps obligatory, such as abandoning heretic innovators and blatant sinners who defiantly disobey Allah if they do not repent. Whoever does this (abandoning a fellow Muslim for more than three days) then dies while insisting on his sin and does not repent before he dies will enter Hellfire. It is known that whoever deserves Hellfire from among the Muslims for a sin that he committed and that Allah did not pardon, then if he enters it, he will eventually come out and will not remain therein eternally, except for the disbelievers, who have no chances of coming out of the Hellfire.