Bede the Venerable — as excerpted in the Catena Aurea on Mark 9:9-13

“(ubi sup.) Again, He will restore all things, that is to say, those things which Malachi points out, saying, Behold, I will send you Elijah the prophet, and he shall turn the heart of the fathers to the children, and the heart of the children to their fathers; (Mal. 4:5, 6) he will yield up also to death that debt, which by his prolonged life he has delayed to render.”