Truly, truly, I say to you, he who believes in Me, the works that I do, he will do also; and greater works than these he will do; because I go to the Father. Whatever you ask in My name, that will I do, so that the Father may be glorified in the Son.
If you abide in Me, and My words abide in you, ask whatever you wish, and it will be done for you. My Father is glorified by this, that you bear much fruit, and so prove to be My disciples. John 15:7-8
To me these are two of the most amazing statements in the Upper Room discourse. In the former, we are told that believers will do greater works than Christ. In the latter, we are told that God wants us to bear lots and lots of fruit. Much could be said about these verses - especially 15:8. Having misunderstood the last clause in Romans 12:3, some have the idea that God predetermines at the beginning of one's Christian life how much faith a person will have to press on in Christlikeness. While a proper reading of Romans 12:3 in it's context would clear the air, John 15:8 will suffice - God's desire for you is that you go a VERY long way in the Christian life!
But actually the thing I want to note about the two passages above is how close prayer is tied in to these statements. The "greater works" of 14:12 happen through the asking of 14:13. Additionally, the multiplied fruit-bearing of 15:8 comes through the prayers of a person who abides in the words of Christ.
Just another reminder that "at all times [we] ought to pray and not to lose heart." (Luke 18:1)