Musannaf Ibn Abi Shayba 19:534

Abu Bakr narrated to us, he said: Abd al-Rahim ibn Sulayman narrated to us, from Abd al-Malik ibn Abjar, from Salamah ibn Kuhayl, from Hujayyah, that Ali used to cut off the hands of thieves, cauterize them, imprison them, and treat them. When they recovered, he would say, "Raise your hands." So they would raise them, and they would look like the penises of donkeys. He would say, "Who cut you?" They would say, "Ali." He would say, "And why?" They would say, "Because we stole." He would say, "O Allah, bear witness. O Allah, bear witness. Now go."