Commentaries:
Robertson's Word Pictures (NT)
By the prophet (dia tou profhtou). The first line is from Isaiah 62:11, the rest from Zechariah 9:9. John (John 12:14 f.) makes it clear that Jesus did not quote the passage himself. In Matthew it is not so plain, but probably it is his own comment about the incident. It is not Christ's intention to fulfil the prophecy, simply that his conduct did fulfil it.
Other Robertson's Word Pictures (NT) entries containing Matthew 21:4:
Matthew 1:22
Mark 11:8
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