Musannaf Ibn Abi Shayba 3:5289

Ibn Fudayl narrated to us, from Ibrahim, the mu'adhdhin of Banu Hanifa, who said: I saw Mahan al-Hanafi, and al-Hajjaj ordered that he be crucified at his door. I looked at him while he was on the wooden beam, and he was glorifying Allah (saying SubhanAllah), magnifying Him (saying Allahu Akbar), declaring His oneness (saying La ilaha illa Allah), and praising Him (saying Alhamdulillah) until he reached twenty-nine, counting with his hand. He was cut [down] while he was in that state. I saw him after a month, and he had twenty-nine [fingers/marks] on his hand, and a light used to be seen with him at night.