What is the ruling on one who always speaks obscenities yet prays and fasts, and when advised, says it is a habit that cannot be overcome? And is there anything in Islamic Sharia that supports this?

Allah has legislated acts of worship to cultivate upright behavior, virtuous morals, and fear of that which displeases the Lord Almighty, for He says: "O mankind, worship your Lord, who created you and those before you, that you may become righteous" and "Indeed, prayer prohibits immorality and wrongdoing" and "O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous."

And whoever's prayer does not restrain him from immorality and wrongdoing will only increase in distance from Allah. For a believer is not one who slanders, curses, is obscene, or speaks ill. And a Muslim should know that the tongue is dangerous, for the Prophet, peace be upon him, took hold of his own tongue when he was asked about what he feared most for himself.

Therefore, it is obligatory to fear Allah the Almighty. Whoever abandons this habit out of fear of Allah, Allah will ease his affairs.