Mark 11:22-24 NKJV
So Jesus answered and said to them, “Have faith in God.  For assuredly, I say to you, whoever says to this mountain, ‘Be removed and be cast into the sea,’ and does not doubt in his heart, but believes that those things he says will be done, he will have whatever he says.  Therefore I say to you, whatever things you ask when you pray, believe that you receive them, and you will have them.
1 Timothy 2:8 NKJV
I desire therefore that the men pray everywhere, lifting up holy hands, without wrath and doubting.
James 1:5-8 NKJV
If any of you lacks wisdom, let him ask of God, who gives to all liberally and without reproach, and it will be given to him.  But let him ask in faith, with no doubting, for he who doubts is like a wave of the sea driven and tossed by the wind.  For let not that man suppose that he will receive anything from the Lord;  he is a double-minded man, unstable in all his ways.