Who called to Abraham from heaven to stop him from harming Isaac?

Answer: The angel of the LORD

"Abraham, Abraham!" called the angel of the LORD. "Do not lay a hand on the boy... now I know that you fear God."

“Just then the angel of the LORD called out to him from heaven, “Abraham, Abraham!” “Here I am,” he replied. “Do not lay a hand on the boy or do anything to him,” said the angel, “for now I know that you fear God, since you have not withheld your only son from me.””— Genesis 22:11-12 (BSB)