2 Thessalonians 1:3:

We are bound to thank God always for you, brethren, as it is meet, because that your faith groweth exceedingly, and the charity of every one of you all toward each other aboundeth;

See also:

What Galatians 2:20 Really Means
How Can My Faith GROW If I Already Have the Fixed Measure of Faith?
Objection: You Can’t Build Up Faith Yourself Because It Is a Gift from God