(Devarim 13:2) "If there arise in your midst a prophet": From (Ibid. 18:15) "A prophet from your midst from your brothers, such as I, shall the L-rd your G-d raise up for you, etc.", (we derive that) just as Moses said "Thus said the L-rd," so, (all to qualify as) "prophets" (must say) "Thus said, etc." And just as Moses spoke and fulfilled part, so, (all) prophets who spoke and fulfilled part. And just as Moses spoke of general and particular, so, (all) prophets. If so, why not say: Just as Moses was eighty years old and the son of Amram, so, (all) prophets! It is, therefore, written "a prophet" — in any event.