Narrated from al-‘Abbas, from Abdullah, from Rifa'ah, who said:
I asked Abu Abdullah عليه السلام, about a person whose hand or foot is amputated - how should he perform ablution? Imam عليه السلام said: "He washes the place where it was amputated."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 1, p. 381