Bukhari Collection: ::Book Number 4:: Hadith # 209
Narrated `Abdullah:Today a man came to me and asked me a question which I did not know how to answer. He said, "Tell me, if a wealthy active man, well-equipped with arms, goes out on military expeditions with our chiefs, and orders us to do such things as we cannot do (should we obey him?)" I replied, "By Allah, I do not know what to reply you, except that we, were in the company of the Prophet and he used to order us to do a thing once only till we finished it. And no doubt, everyone among you will remain in a good state as long as he obeys Allah. If one is in doubt as to the legality of something, he should ask somebody who would satisfy him, but soon will come a time when you will not find such a man. By Him, except Whom none has the right to be worshipped. I see that the example of what has passed of this life (to what remains thereof) is like a pond whose fresh water has been used up and nothing remains but muddy water."