The Conduct of Spiritual Aspirants and Understanding the States of Worshippers, in the chapter on the whisperings Satan brings to those who repent:

“As for the third approach he uses against those who repent, when they resist him and seek protection in Allah: he whispers to them concerning the Creator, seeking to corrupt the foundations of their belief in His oneness.”

He continued at length until he said:

“This is among his gravest whisperings: casting doubt upon Allah’s oneness; likening and comparing the Lord’s attributes to those of creation; denying and negating them; or introducing standards that measure the Lord’s greatness according to the limits of their intellects. They are ruined if they accept this, and their foundations are shaken unless they seek refuge from it in knowledge and a sound understanding of Allah, Mighty and Majestic, based on what He has said about Himself, how He has described Himself, and how His Messenger has described Him.”

He continued until he said:

“He, exalted is He, is the One who said, ‘I am Allah’—not the tree; the One who comes after not having come before—not His command; the One who rose over His Throne in His greatness and majesty, to the exclusion of every other place; the One who spoke directly to Moses and showed him mighty signs of His, so that Moses heard Allah’s speech; the One who outlasts His creation, who hears their voices and looks with His eye upon their bodies. His two hands are outstretched, and they are distinct from His favour and His power. He created Adam with His two hands.”

And he mentioned other things as well.