From Sahih Bukhari (Hadith 2978):
Narrated Ibn `Abbas:Abu Sufyan said, "Heraclius sent for me when I was in 'Ilya' (i.e. Jerusalem). Then he asked for the letter of Allah's Messenger (ﷺ) and when he had finished its reading there was a great hue and cry around him and the voices grew louder and we were asked to quit the place. When we were turned out, I said to my companions, 'The cause of Ibn Abi Kabsha has become conspicuous as the King of Bani Al- Asfar is afraid of him
Reference: Bukhari 2978