Charles Ellicott Commentary Genesis 21:17

"And God heard the voice of the lad. And the angel of God called to Hagar out of heaven, and said unto her, What aileth thee, Hagar? Fear not. For God hath heard the voice of the lad where he is." — Genesis 21:17 (ASV)

The angel of God. —In Genesis 16:7 it was “the angel of Jehovah” which appeared to Hagar; here it is the angel of Elohim. It is impossible not to be struck with this exact use of the names of Deity. Hagar was then still a member of Abraham’s family; here she is so no longer; and it is Elohim, and not Jehovah, the covenant God of the chosen race, who saves her.