I believe the quote you are referring to is from the Alfred Tennyson poem "The Charge of the Light Brigade." The quote goes: Theirs not to reason why, Theirs but to do and die...
""Forward, the Light Brigade!"
Was there a man dismay'd?
Not tho' the soldier knew
Someone had blunder'd:
Their's not to make reply,
Their's not to reason why,
Their's but to do and die:
Into the valley of Death
Rode the six hundred.