Where in the Old Testament does Les find another reason to connect Elijah with the two witnesses?
The Old Testament passage is Malachi chapter 4, verse 5. There God promises to send Elijah the prophet before the great and dreadful day of the Lord. This supports identifying one of the two witnesses as Elijah, in addition to Elijah having been taken up without dying.
Les explains that after discussing the two witnesses in Revelation chapter 11, he points back to Malachi 4:5 as another reason to connect Elijah with them. Malachi 4:5 says, “Behold, I will send you Elijah the prophet, before the coming of the great and dreadful day of the Lord.” Les notes this is plain English: before the great and terrible day of the Lord comes, Elijah will come. Verse 6 adds that Elijah will turn the heart of the fathers to the children and the children to their fathers, lest the earth be smitten with a curse. This gives another scriptural basis for expecting Elijah as one of the two witnesses.
Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD: