‘Umm al-Mu’minin A’ishah Siddiqa (may Allah be pleased with her) says that the Messenger of Allah (peace and blessings be upon him) said: No one should say that my soul has become impure (i.e., filthy and unclean), rather he should say that my soul has become lazy and sluggish. (Impure and unclean is a title for a disbeliever, and therefore a Muslim is forbidden from using this word for himself. And in another hadith it is mentioned that then in the morning the impure soul rises, so that is the attribute of someone else and refers to an ambiguous person; such usage is not prohibited.)