Question:
What Scripture do we have to say that prophecy is speaking a word from God for the time? Why do we define prophecy that way?

Answer:
When we think of a prophet in the Old Testament, we think of him foretelling events, but most Old Testament prophets really did speak a word for the time. This is what prophecy has always been:

“Sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear” (1 Peter 3:15).“If any man speak, let him speak as the oracles of God; if any man minister, let him do it as of the ability which God giveth: that God in all things may be glorified through Jesus Christ, to whom be praise and dominion for ever and ever. Amen” (1 Peter 4:11).