Musnad Ahmad 1060:31

Sufyan narrated to us, from al-Zuhri, from ‘Ata’ ibn Yazid, from Abu Ayyub, mentioning therein the Prophet, may Allah bless him and grant him peace: "It is not lawful for a Muslim to forsake his brother for more than three [nights]. They meet, and this one turns away, and that one turns away; and the best of them is the one who initiates the greeting."