مُسْنَدُ أَحْمَدَ
Musnad Ahmad 1775
Musnad Bani Hashim (Hadeeth of al-'Abbas bin 'Abdul-Muttalib from the Prophet)
It was narrated that Katheer bin ‘Abbas said: ‘Abbas and Abu Sufyan were with him - meaning the Prophet (ﷺ) - when he addressed them and said, "Now the battle is raging.” He said: “Call out: O people of Soorat al-Baqarah."
Musnad Ahmad 1776Sahih (Darussalam) [ Muslim (1757)]