For nothing can be in itself. But the substance or nature of God—i.e. the Godhead—is said to be in God. Therefore it seems that God is not the same as His essence or nature.
 Thomas Aquinas, Summa Theologica (1274). copy citation

add
Author Thomas Aquinas
Source Summa Theologica
Topic God essence
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17611/pg17611-images.html

Context

“Hence it does not follow that matter exists in God. _______________________
THIRD ARTICLE [I, Q. 3, Art. 3]
Whether God is the Same as His Essence or Nature?
Objection 1: It seems that God is not the same as His essence or nature. For nothing can be in itself. But the substance or nature of God—i.e. the Godhead—is said to be in God. Therefore it seems that God is not the same as His essence or nature. Obj. 2: Further, the effect is assimilated to its cause; for every agent produces its like. But in created things the suppositum is not identical with its nature; for a man is not the same as his humanity.” source