Deuteronomy 18:18: I will raise up for them a Prophet like you from among their brethren, and will put My words in His mouth, and He shall speak to them all that I command Him. 

The Biblical doctrine of the true purpose and function of the prophets and their manner of delivering the message is clearly set forth in the Lord's words to Moses. Jehovah would speak not so much to the prophets as He would through them, but no others. They were to speak precisely the words given to them, but no others. "I have put my words in thy mouth," the Lord said to Jeremiah in appointing him a prophet to the nations (Jer 1:9). Identically the same words were spoken to Isaiah (51:16; 59:21), and the formula, "Thus saith Jehovah," is repeated some eighty times in the book of Isaiah alone. Even the false prophet Balaam could speak only that which Jehovah gave him to speak--"And the angel of Jehovah said unto Balaam, Go with the men; but only the word that I shall speak unto thee, that thou shalt speak" (Nu 22:35; 23:5, 12, 16). In many Old Testament passages it is nothing other than a process of "dictation" which is described, although we are not told what the method was by which this dictation was accomplished. In others we are simply given to understand that Jehovah spoke through chosen men as His organs, were His words and were distinctly superhuman product. The uniform teaching of the Old Testament is that the prophets spoke when, and only when, the word of Jehovah came to them: Hosea 1:1; Amos 1:3; Micah 1:1; Malachi 1:1, etc
The characteristic word for prophet is nabhi, "spokesman," is not merely spokesman in general, but by way of eminence, that is, God's spokesman, In no case does the prophet presume to speak on  his own authority. in the first place is not of his own choosing, but is a response to a call from God., oftentimes a call which was obeyed only with reluctance; and he speaks or forbears to speak as the Lord gives him utterance.  --Loraine Boettner "Studies in Theology" p17-18