Musannaf Ibn Abi Shayba 1:2106

Abu Bakr narrated to us, he said: Sahl ibn Yusuf narrated to us, from ‘Amr, from al-Hasan, regarding the one whose hand is severed: If his hand is severed from the joint, and he wants to perform ablution, he washes the severed part. And if the palm is severed, he washes up to the elbow.