Musnad Ahmad 1060:87

Rawh narrated to us, Malik and Salih narrated to us, from Ibn Shihab, that ‘Ata’ ibn Yazid narrated to him, from Abu Ayyub, from the Prophet (peace and blessings of Allah be upon him) that he said: "It is not lawful for a Muslim to forsake his brother for more than three [nights/days], such that they meet, and this one turns away, and that one turns away. And the best of them is the one who initiates the greeting."