And it came in the epistle The Guidance and Directing (Risalat Hidayah wa Irshad):

"Whosoever worships the form on the basis that it is bounded, possessing length and width like his own form, has asserted corporeality and likeness. Whosoever worships the Name without the Essence (al-Ma'na) has worshipped a originated thing, because the Essence is prior to the Name and prior to the attributes. And whosoever claims that he worships the Name, the Essence, and the form together, has worshipped many partnered lords and bounded deities."

And it came in The Reminder for Seekers (Tadhkirat al-Muridin):

"And the manifestation of divine power necessitates the negation of physical form."

And our Master ar-Rida, glorified is his glory, said:

"Verily, that which you witnessed with your own eyes is Allah in relation to His power, and it is not Allah in relation to physical form. Thus, when miracles manifest, the physical form becomes void; because if that form were truly His form in reality, He would not be able to manifest miracles.