It is not necessary to recite the Kalimah during Ghusl.
Some people say that it is necessary to recite the Kalimah while performing Ghusl (ritual bath), otherwise the impurity is not removed. Similarly, some consider it necessary to recite the Kalimah while bathing a dead body; according to Shariah, both of these things are not proven. Rather, reciting the Kalimah or doing any other Dhikr (remembrance of Allah) while performing Ghusl has been prohibited. (Aham Masail/Vol:8/Page:77)