Musnad Ahmad 1234:3

Yahya ibn Sa'id narrated to us, from Isma'il, who said: Qays narrated to us, saying: I came to Khabbab to visit him, and he had been cauterized seven times on his stomach. I heard him saying: "Were it not that the Messenger of Allah (peace and blessings of Allah be upon him) forbade us from praying for death, I would have prayed for it."