Musannaf Ibn Abi Shayba 36:58

Waki’ narrated to us, he said: Sufyan narrated to us, from Abu Ishaq, from Abu Layla al-Kindi, who said: Khabbab came to Umar, and he (Umar) said: "Draw near, for there is no one more deserving of this seat than you, except for Ammar." He said: Then Khabbab began to show him marks on his back from what the polytheists had tortured him with.