Mark 9:11-13

9:11 Then they asked him, “Why do the experts in the law say that Elijah must come first?” 9:12 He said to them, “Elijah does indeed come first, and restores all things. And why is it written that the Son of Man must suffer many things and be despised? 9:13 But I tell you that Elijah has certainly come, and they did to him whatever they wanted, just as it is written about him.”

Malachi 4:5

4:5 Look, I will send you Elijah the prophet before the great and terrible day of the Lord arrives.