Reference for Arakhin 38:4

משקל ידי ומשקל רגלי עלי רבי יהודה אומר

Our Rabbis taught: [If one said:] 'I vow the weight of my hand and the weight of my foot', R'Judah says: Let him bring a barrel, fill it with water, place his hand therein up to the elbow, and his foot up to the knee; then let him weigh the flesh, bones and sinews of an ass and put it in [to the barrel] until it is filled up.

Jastrow