The phrase God of heaven is a name for the Lord used during and after the exile (when many Israelites were taken from their land). It appears in:
Ezra (Ezra 1:2; 5:11, 12; 6:9, 10; 7:12),
Nehemiah (Nehemiah 1:5; 2:4, 20), and
Daniel (Daniel 2:18, 19, 37, 44).