Hisham ibn Hakam and Hisham ibn Salim were Mujassimas.
Here are the traditions Shaykh Saduq quotes in his book on Tawhid.
H 280, CAh. 11, h 5
Ali ibn Muhammad, in a marfu‘ manner, has narrated from Muhammad ibn al-Faraj al-
Rukhkhaji who has said the following. "I wrote to Imam Abu al-Hassan (a.s.), about the words of Hisham ibn al-Hakam, about the body and the words of Hisham ibn Salim about the form (of Allah). He wrote in reply, ‘Remove from thyself the confusion of the confounded people and seek refuge from Allah against Shaytan (Satan). What the two Hishams have said is not correct. (al Kafi)
The translator places a note claiming that this ahadith is rated marfu in traditional sciences. Next he tells the reader to refer to the commentary of Usul al –Kafi by Mawla Salil Mazindarani vol 3 pg288
H 276, Ch. 11, h 1
Ahmad ibn Idris has narrated from Muhammad ibn ‘Abd al-Jabbar from Safwan ibn Yahya from Ali ibn abu Hamzah, who has said the following.
"I stated before abu ‘Abdallah (a.s.) that I have heard Hisham ibn al-Hakam quoting you that Allah is a body of Self-subsisting nature and is from light. He can very clearly be recognized and He bestows such knowledge to whoever among His creatures He wills." The Imam said,
"Glorious is He, Whom no one knows how He is except He Himself. There is no one similar to Him and He is All-hearing, All-seeing. He cannot be limited, nor can He be felt or touched or moved. Eyes cannot see Him nor any of the senses can comprehend Him. He cannot be contained in anything, nor has He any body or form or figure or confine." (al-Kafi)
Ali bin Ahmad ibn Muhammad ibn Imran al Daqqaq said: Muhammad ibn Abu Abd allah al Kufi said: Muhammad ibn Ismail al Barmaki said: on the authority of al-Husyan ibn al Hasan, and al Husayn ibn Ali on the authority of Salih ibn Abu Hammad, on the authority of Abd Allah ibn Al Mughayrah, on the authority of Muhammad ibn Ziyad said: I heard Yunus ibn Zabyan say
I entered in the presence of Abu Abdullah al Sadiq (as) and told him:
Hisham ibn al Hakam made a long statement. I will simply summarize it. He considers Allah to be a body because things are two types: bodies and actions of bodies. Hence, if it not possible for the Creator to be an action, it is only possible that he is Doer.
Then Abu Abdullah (as) replied: “Woe to him! Does he not know that a body and an image is extremely limited. Hence, if a limit is possible then increase and decrease are possible, then He is created.”
Ali bin Ahmad ibn Muhammad ibn Imran al Daqqaq said: Muhammad ibn Abu Abd allah al Kufi said: Muhammad ibn Ismail al Barmaki said: on the authority of Ali bin Al Abbas, on the authority of al-Hasan ibn Abd Al-Rahman al Hammani that
I said to Abu Al Hasan Musa ibn Jafar (as), Hisham ibn Al Hakam thinks that Allah is a body. There is nothing whatsoever like unto Him, the All-Knowing, the All-Hearing, the All-Seeing, the Omnipotent, the Speaker, and the Communicator. Speech, Omnipotence and Knowledge flow from the One same source, None of these things are created.
Imam Musa Al Kazim (as) replied “May Allah Kill Him (Hisham ibn Hakam) ! Does he not know that a body is limited, and the speech is other than then the speaker? Allah forbid, I am free from such words. He is devoid of both body and image. He has no limits, and everything other than Him is created. Verily, everything that exists is the result of his Will and Intent which is expressed without words, the use of the tongue, or any reluctance or hesitation.
Muhammad ibn Musa ibn Al Mutawakkil said “Ali ibn Ibrahim ibn Hishim said on the authority of his father, on the authority of al Saqr ibn [Abu] Dulaf that
I asked Abu Al Hasan Ali ibn Muhammad ibn Ali Ibn Musa Al Rida (ra) about Divine unity: Verily, I say what Hisham ibn Al Hakam said:
So he (as) became angry and said:
What is the matter with you and Hisham? Indeed, he is not from us who thinks that Allah the Mighty and High has a body. We are free from such a person in this world, and in the Hereafter. On son of [Abu ] Dulaf! Verily, the body is created and Allah is its Creator.