Charles Ellicott Commentary Matthew 17:23

"and they shall kill him, and the third day he shall be raised up. And they were exceeding sorry." — Matthew 17:23 (ASV)

They were extremely sorry—St. Mark (Mark 9:32) and St. Luke (Luke 9:45) add that they understood not the saying; it was hid from them, that they should not perceive it; and they were afraid to ask Him. Their sorrow was vague and dim, and they shrank from what might make it more definite.