WE BELIEVE that all the prophets are INNOCENT through out their lives. this means that by the GRACE of GOD they are always guiltless, before and after their prophetic mission. If not so; people would lose their confidence on them, and would not know them good mediators to act between them and God.
If in the Holy QURطN, guilts are ascribed to them, it is only:_ PREFERRING THE BETTER OVER THE BEST. In other word, they may choose one of the two which is less good, and not that which is bad. Some times a good work of a good believer might be a sin for the nearest to God: and every one is expected to do what is becoming to HIM.