Usul al-Kafi
Book 3: The Book of Excellence of Knowledge

Chapter 5
The One who deserves to be worshipped

169. 2. Hisham Ibn Hakam: From which word Allah (God) has been derived?
Imam Jafar as-Sadiq (p): O Hisham, the word Allah (God) is a derivative of the root word ilah which means ‘one who is worshipped’, and the one who is worshipped must necessarily be worthy of being worshipped and the name (of the object) is different from the object itself. Thus whoever worshipped the name rather than its meaning has become a heathen and in reality has worshipped nothing. And he who has worshipped the name and the meaning together has also gone astray and has worshipped duality. And whoever worshipped the meaning (of the name of God) rather than the names, has worshipped Unity. This in fact is the Unity of God. O Hisham, have you grasped this point?
Hisham: Add to my understanding.
Imam Jafar as-Sadiq (p): God has ninety-nine names. Had the name and the named one been the same, there would have been one God for each and every name. But God is the meaning of what these names stand for. All the names are apart from Him. O Hisham, bread is the name of what is eaten, and water is the name of what is drunk, and clothe is the name of what is worn and fire is the name of what burns.
O Hisham, have you now understood enough to enable you to debate with our opponents and also with those who join others in their worship of God, the Glorified and the Almighty?
Hisham: Yes.
The Imam (p): May God reward you for this and keeps you steadfast in it…