Musnad Ahmad 21:13

Sufyan narrated to us, he said: I heard al-Zuhri—once or twice, but I did not memorize it—from Kathir ibn Abbas, who said: Abbas was there, and Abu Sufyan was with him—meaning the Prophet, peace and blessings of Allah be upon him. He said: So he addressed them and said: "Now the furnace has heated up." And he said: "Call out: 'O companions of Surah al-Baqarah!'"