Luke 5:5

And Simon answered and said, Master, we toiled all night and took nothing: but at thy word I will let down the nets.

Peter's objection against the thing Jesus commanded was well founded from the earthly viewpoint. It was not a good time to fish; the men were tired; they were cleaning up; and it could not have been an altogether welcome command from Jesus, who said, in effect, "Come on, let's go fishing!" Peter's response here, while obedient, was clearly petulant, and not spontaneous at all. Grudgingly agreeing to do it, he nevertheless made his displeasure known.