It is mentioned in Ayah 84 of Surah Zukhruf:

“It is Allah (SWT) who is God in heaven and on earth; God is the All Wise, the All Knowing.” That is why God is with everything and is in everywhere, but according to Imam Ali (AS) is not mixed (with anything) [1], rather accompanies and this means God gives life to everything. Therefore, God is nearer to us than ourselves, but you do not see God, as is mentioned in Ayah 85 of Surah Waqi’ah: “God is nearer to him/her than you, though you do not see God.” And also in Ayah 24 of Surah Al-Anfal it is said: “Know that God comes between a human being and his/her heart.”

[1] Nahul Balaghah, Sermon 1.

Source: en.shafaqna.com