The proof of this characteristic of God is found throughout the Bible, but it seems that not many pastors preach specifically about it, and too few Christians actually think much about it. Maybe it is because our limited, finite human minds struggle to grasp the vastness and greatness of God. I too struggle and surely do not have everything figured out.
Omnipresent means present everywhere at the same time. The Bible teaches that God is everywhere at all times. That means there is no place you can go that He is not there, and there is no time you can be in where He is not there.