All Praise is due to Allah Alone and peace and blessings be upon His Messenger, his family and Companions.
It is Makruh (disliked) to mention Allah's Name or say Tahlil in the bathroom and it is not lawful to say the Shahadah while pouring water over the body during Ghusl following Janabah. However, it is a Sunnah (a commendable act) for those entering the bathroom or a place to relieve themselves to seek refuge with Allah from evil and evildoers before entering, and to say after leaving: "I ask You (O Allah!) for forgiveness," and when leaving after taking the Ghusl following Janabah to say: "I bear witness that there is no deity except Allah Alone, without partner, and I bear witness that Muhammad is His Servant and Messenger. O Allah! Make me of those who return to You often in repentance and those who remain pure." This is what has been authentically reported from the Prophet (peace be upon him).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.