Juan 4:38

I sent ye to reap that whereon ye have not labored: others have labored, and ye are entered into their labor.

This was a stern reminder to the apostles that the great ingathering they were about to see was in no sense the result of their own efforts and abilities, and that they were to consider themselves instruments of God in reaping the fruit of the labors of others, in this case, the labor of the Master himself, and of the woman. This was the viewpoint expressed by Paul, as cited above. So great a response to the gospel as the apostles were about to see might easily have turned their heads except for the Saviour's warning here.