6 “These things understood not His disciples at the first; but when Jesus was glorified,” that is, when He had manifested the power of His resurrection, “then remembered they that these things were written of Him, and they had done these things unto Him,” that is, they did nothing else but what had been written concerning Him. In short, mentally comparing with the contents of Scripture what was accomplished both prior to and in the course of our Lord's passion, they found this also therein, that it was in accordance with the utterance of the prophets that He sat on an ass's colt.
Source: Tractates on the Gospel of John (New Advent)