The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wa'l-Adab)
Muslim :: Book 32 : Hadith 6212
'Abdullah b. 'Umar reported Allah's Messenger (may peace be upon him) as saying: It is not permissible for a Muslim to have estranged relations with his brother beyond three days.