Which is permissible to say: "You cannot, by God" or "By God, you cannot"?

If an oath is an affirmation of a negative answer, such as "No, by Allah," then there is no harm in it. However, one should not frequently swear by Allah, especially if the swearer is not certain of fulfilling their oath, due to Allah's saying: "And guard your oaths" and "And do not make Allah an object for your oaths."