The places of worship (prostration) are for Allah

Zarqan, the close friend of Abu Dawod, said, ‘One day, he (Abu Dawod) came back from al-Mu'tasim distressedly. I asked him what the matter was and he said, ‘A thief confessed that he had committed theft and the caliph wanted to purify him by punishing him. He gathered the jurisprudents in his meeting and sent for Muhammad bin Ali (al-Jawad). He asked us how the thief’s hand would be cut. I said, ‘His hand should be cut from the wrist because Allah has said when talking on Tayammum, (…and wipe your faces and your hands). Some jurisprudents agreed with me on that and others said, ‘The hand must be cut from the elbow.’ Al-Mu'tasim asked what their evidence was and they said, ‘Allah has said, (…wash your faces and your hands as far as the elbows…). Then, al-Mu'tasim turned to Muhammad bin Ali and asked him, ‘O Abu Ja'far, what do you say on that?’
He said, ‘O Ameerul Mo’mineen, the jurisprudents have talked about that.’
Al-Mu'tasim said, ‘Let what they have said aside. What do you have to say?’
Imam al-Jawad (a.s) said, ‘O Ameerul Mo’mineen, would you exempt me from that?’
Al-Mu'tasim said, ‘I adjure you by Allah that you tell what you have.’
Imam al-Jawad (a.s) said, ‘Now, since you have put me on oath, I will say. They (jurisprudents) are mistaken. Only the fingers should be cut from the joints of their origins and the palm should be left safe.’ Al-Mu'tasim asked about the evidence and Imam al-Jawad (a.s) said, ‘The messenger of Allah said, ‘Prostration is on seven organs; the face, the two hands, the two knees and the two feet.’ Then, if his hand is cut from the wrist or the elbow, he shall not have a hand to prostrate on it and Allah has said, (the places of worship (prostration) are for Allah) meaning these seven organs that one prostrates on; (so pray not unto anyone along with Allah). What is for Allah is not cut.’ Al-Mu'tasim approved this answer and ordered the hand of the thief to be cut from the joints of fingers without the palm.( Tafsir al-Ayyashi, vol.1 p.319, al-Burhan, vol.1 p.471, Bihar al-Anwar, vol.12 p.99, Wassa’il ash-Shia, vol.18 p.490.)