Deuteronomy 18:22 (Read all of Deuteronomy 18)

When a prophet speaketh in the name of the Lord, if the thing follow not, nor come to pass, that is the thing which the Lord hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.

That is God's spoken words and not man, we need to try the spirits and know whether their are of God or not. Sometimes when God begin to use His people, they can become prideful and if they don't be careful can do things in self rather than the Holy Spirit using them.