The same words appear in Matthew 12:35, but were probably spoken there on a different occasion. There is no doubt that the Lord used the same illustrations – sometimes expressed slightly differently – on different occasions – see for example Matthew 7:16-20 and 12:33.
‘For of the abundance of the heart his mouth speaketh.’ You cannot go any further. You're stuck. You may even have got this far. You may have decided to pursue God with a spiritual appetite, to take seriously the message of the gospel. You may have been prepared to see your own sin, but beyond that, you cannot go, because the only person who can bring forth fruit to God or get near to God or please God is someone whose heart is right, and yours isn't. You cannot do anything about that. But praise God, you can go further. There is only one way you can do so: by recognizing that God must change your heart. God must take the old nature out and put a new nature in. God himself must work within you. Didn't you know that this is what conversion is about? This is what it's about. This is what Christ’s disciples are preaching, that a man becomes a Christian because God changes him. God brings his soul to life. God takes out the unregenerate heart, the unbelieving, selfish, hard ways and puts a new nature within him. That is what makes a man a Christian. ‘Marvel not that I said unto thee,’ he said to Nicodemus of old, ‘Ye must be born again’ (John 3:7). No man can enter into the kingdom of heaven, he says, unless he is born from above, born again.