Musnad Abu Ya'la al-Mawsili 150:79

Abu Musa narrated to us, Muhammad bin 'Uthmah narrated to us, 'Abdullah bin Munib—meaning al-Madani—narrated to us, Hisham bin 'Urwah informed me, from 'Urwah, from 'Aishah, that the Messenger of Allah (ﷺ) said: "It is not for a Muslim to forsake a Muslim for more than three days. So if he meets him and greets him three times, and he does not return the greeting to him, then he has returned with his sin along with his [previous] sin."