Matthew 14:31 (NIV) Immediately Jesus reached out his hand and caught him. "You of little faith," he said, "why did you doubt?"
James 1:5-8 (NIV) 5 If any of you lacks wisdom, he should ask God, who gives generously to all without finding fault, and it will be given to him. 6 But when he asks, he must believe and not doubt, because he who doubts is like a wave of the sea, blown and tossed by the wind. 7 That man should not think he will receive anything from the Lord; 8 he is a double-minded man, unstable in all he does.
Romans 4:20-21 (NIV) 20 Yet he did not waver through unbelief regarding the promise of God, but was strengthened in his faith and gave glory to God,21 being fully persuaded that God had power to do what he had promised.
Mark 11:23 (NIV) "I tell you the truth, if anyone says to this mountain, 'Go, throw yourself into the sea,' and does not doubt in his heart but believes that what he says will happen, it will be done for him.
We are all impatient of uncertainty, either in opinion or in conduct; but if you are not quite sure what God wants you to do, you may be quite sure that he does not at present want you to do anything. Alexander Maclaren (1826-1910) Draper's Book of Quotations for the Christian World.