Praise be to Allah,
Sufyaan ibn ‘Uyaynah narrated to us from [Muhammad ibn Shihaab] az-Zuhree from Anas ibn Maalik who said that Allah’s Messenger (PBUH) said,
“Do not desert one another, do not hate one another, do not turn away from one another, and do not envy one another, and be, slaves of Allah, brothers. It is not permissible for a Muslim to boycott his [Muslim] brother for more than three days.”
(Musnad Ahmad, Vol. 3, p. 110. The isnaad of this hadeeth is Saheeh according to the conditions set by al-Bukhaaree and Muslim.)
Source:
Thulaathiyyaat Musnad Ahmad ibn Hanbal (may Allah have mercy on him)
Reports of Imaam Ahmad that have only three narrators between him and the Prophet, peace, and blessings be upon him.
Shaykh Muhammad ibn Ahmad ibn Saalim as–Saffaareenee al–Hanbalee ~(1114 –1188 AH)