In the Holy Qur'an

Say: He, Allah is One. Allah is He upon whom all depend. He begets not, nor is he begotten. And none is like unto Him (112:1-4)

In the Traditions

Imam Ali was asked by one of his companions, Dhi'lib al Yamani, "Have you see your Lord" The Imam replied, "I would not worship a Lord whom I have not seen." He was then asked, "How did you see Him?" The Imam replied, "The eyes cannot see Him according to outward vision, it is the hearts that perceive Him, through the verities of faith."