When he washed the body of the Prophet (may Allah's peace and blessings be upon him) he looked for what is usually found on the dead person’s body (i.e. dirt), but found nothing. Therefore, He said: May my father be sacrificed for you, you are pure alive and dead

When ‘Ali (may Allah be pleased with him) washed the body of the Messenger of Allah (may Allah’s peace and blessings be upon him) he went on looking for things that are usually looked for on the deceased like urine, excrement, and the like. Such things usually come out of the corpse as a result of joints’ relaxation. Nevertheless, ‘Ali (may Allah be pleased with him) found nothing.

So, he said: May my father be sacrificed for you, you are the pure one; you were pure when alive and you are pure when dead.