Seeing Allah is Impossible

It is impossible for men to see Allah, the Most High; it was spread among the people of that time that Mohammed, may Allah bless him and his family, had seen his Lord, hence Ibrāhim b. Mohammed al-Khazzāz and Mohammed b. al-Husayn hurried to Imām al-Ridā, peace be on him, to ask him about that: “Did Mohammed, may Allah bless him and his family, see his Lord in the form of a perfect youth with His hair hanging down on the lobs of His ears and of a man in the age of thirty?”
They ascribed this statement to a group of the Shi‘ite great figures, and that they said: “The body of Allah right up to the navel was quite empty and the rest of Him was full.” Without doubt this statement was fabricated and attributed to such a group of Shi‘ite eminent men, for they had no relationship with such forbidden things. As for the Imām, he disordered when he heard this false statement. He prostrated himself in prayer and began addressing Allah, the Exalted, humbly: “Glory belongs to You! They do not know you; nor do they know You as a Unity. For this reason they have described You (with fabricated attributes). Glory belongs to You! Had they known You, they would had ascribed to You only the attributes You have ascribed to Your Own Self. Glory belongs to You! How did they allow themselves to listen to anything about You other than Your Self.
“O Allah, I do not describe You but through what You have described Your Own Self; nor do I liken You to Your creatures. You are the Possessor of all good. So do not place me among the unjust people.”
Then he, peace be on him, turned to them and said to them: “After whatever you imagine, imagine that Allah is other than that.”
Namely, whatever man imagines concerning the Essence of Allah, the Most High, he should imagine that Allah is other than what he has imagined.
Then the Imām turned to Mohammed b. al-Husayn and said to him: “We, the family of Mohammed, represent the moderate path. Our path is lost to those who exceed the limit, and those who lag behind cannot come unto our (path). O Mohammed (b. al-Husayn), verily, when Allah’s Messenger, may Allah bless him and his family, looked at the Majesty of his Lord, he was in the form of a youth in full bloom and of a man in the age of thirty with his hair hanging down on the lobes of his ears. O Mohammed, Allah is far above having the attributes of His creatures.”
“Who was he whose feet were on the green yard?” asked Mohammed b. al-Husayn.
“He was Mohammed,” replied the Imām, “When he looked at his Lord through his heart, He established him in His Light which was like the Light of the (Divine) veil, until what was behind the (Divine) veil was made manifest to him. Verily, the effulgence of Allah’s Light is green, red, white, and the like. O Mohammed, we (the Imāms) only say what the Book (of Allah) and theSunna bear witness to. ( Usūl al-Kāfi, vol. 1, p. 101-102 )

Reference :The life of Imām'Ali Bin Mūsā al-Ridā