Is it permissible to lie on one's back, provided that one does not place one leg over the other, given the contradiction between the hadith "He [the Prophet] forbade ishtimal as-Samma' and for a man to raise one of his legs over the other while he is lying on his back" and the hadith "I saw the Messenger of Allah, peace and blessings be upon him, in the mosque placing one of his legs over the other"?

It is forbidden to lie on one's back and raise one leg over the other if this leads to the exposure of the 'awrah (private parts); this is due to the Prophetic prohibition reported by Jabir ibn Abdullah.

However, if the exposure of the 'awrah is assured not to occur, then there is no harm in it; this is because the Prophet, peace and blessings be upon him, did so in the mosque, and because the default rule for matters is permissibility unless a prohibition is proven by an uncontradicted دليل (evidence).

It is preferable to reconcile the narrations regarding this issue, whereby the prohibition is understood to apply to the case of 'awrah exposure, and the Prophet's action, peace and blessings be upon him, is understood as an explanation of permissibility when its exposure is assured not to occur. One should not resort to claiming abrogation due to the lack of established chronology.