Sahih Muslim
The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wal-Adab)
Book Number: 032 / Hadith Number: 6213
'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.