Question: I can remember me a hadeeth/report of a sahabiy that took a shower before profession the shahaadatayn. What does this mean?
Answer: It is possible that he or she became Muslim before that secretly, and that the wash was for the announcement. It is only a story about an action that was happened.
In any case, to delay becoming Muslim by taking a wash only increases the time one remains a non-muslim. One becomes Muslim by pronouncing the shahaadatayn, in any language, and in any phrase that has the meaning "there is no god but God, and Muhammad is His prophet and messenger", while intending to become Muslim.
Becoming Muslim is an immediate obligation for any non-Muslim, so how can it be allowed to delay this for the purpose of cleanliness? Moreover, the scholars agree that worship is not accepted from a non-Muslim. Being Muslim is a prerequiste for accepted worship, so taking a wash before become muslim is and utter waste of time and a blashpemous delay of the most important obligation upon a human being.
Based on this, if someone came to a Muslim and said, "I want to become Muslim" he must immediately tell him, "With the intention to become Muslim, say 'There is no god but God, and Muhammad is His prophet and messenger.'" If he instead said, "Go and take a shower," or delayed him in any other way, he has committed blasphemy and is no longer a Muslim. This is because he was the direct and willful cause of the other person remaining a non-Muslim for some time.