The Messenger of Allah ﷺ said, "It is not lawful for a Muslim to shun his brother for more than three days."
حدثنا يحيى بن آدم حدثنا إسرائيل عن أبي إسحاق
قال رسول الله ﷺ لا يحل لمسلم أن يهجر أخاه فوق ثلاث
Its isnad is sahih. It is found in Majma al-Zawaid 8:66, where it is stated, "Ahmad, Abu Yaala, Bazzar, and Tabarani narrated it, and the men of Ahmad's chain are the men of the Sahih."