Though we heed His teaching in the Gospels, we can forget that Jesus is a prophet. In fact, He is the prophet, par excellence. As the Word of God (John 1:1), He speaks, by the Spirit, throughout Scripture. Some people believe that Jesus has nothing to say about topics never mentioned in the Gospels, even if they are discussed in other biblical books. But all the doctrines of the Bible are the doctrines of Jesus, even if they are not directly recorded in the Gospels.
For further study:
The Bible in a year: