If you want your tefillos to be accepted by Hakadosh Baruch Hu, be cautious with your mouth, which is your utensil for prayer.  Be cautious that you shouldn’t speak profane language, curses, swears, lashon hara, and the like, because prayer is meant to give a gift to Hakadosh Baruch Hu, and the mouth is the vessel that holds that gift, and it shouldn’t be dirty… Otherwise, it is a disgrace for the King, and a disgrace for the gift.

If a person wasn’t careful with his mouth, he should correct that, and stop speaking in such a manner. He should speak Torah and tefillah and permitted speech, and nothing else. ~ The Kitzur Shlah