Musannaf Ibn Abi Shayba 27:607

Waki’ narrated to us, he said: Sufyan narrated to us, from Abu Ishaq, from Ibn Abi Layla al-Kindi, who said: Khabbab came to Umar and said: "Bring him near." [Umar said]: "No one is more deserving of this seat than you, except for Ammar." Then Khabbab began to show him the scars on his back from how the polytheists had tortured him.