In Luke 11:50-51, the Lord Jesus Christ calls Abel a prophet:
"Therefore this generation will be held responsible for the blood of all the prophets that has been shed since the beginning of the world, from the blood of Abel to the blood of Zechariah, who was killed between the altar and the sanctuary. Yes, I tell you, this generation will be held responsible for it all." Luke 11:50-51 NIV
I have in mind the concept of prophet as one who acted as God's mouthpiece, relaying the words of God to men in accordance with God's instructions. However, I coun't find no record of any prophetic utterance from Abel.
What then made him a prophet?