What is the ruling on a Muslim sitting with one foot placed over the other, and is there a sound prophetic prohibition concerning this sitting posture?

There is no prohibition against sitting with one leg crossed over the other. However, there is a prohibition regarding lying down and raising one leg over the other, though there are also reports that contradict this. Scholars have reconciled these by explaining that the prohibition applies when the 'awrah (private parts) are exposed, while it is permissible when safety from exposure is assured. The Prophet (peace be upon him) did it, and the prohibition is either abrogated or understood to apply out of fear of 'awrah exposure, while permissibility applies when such exposure is safe.