Is placing the right hand over the left in prayer obligatory, based on Sahl ibn Sa'd's statement, "People were commanded to place the right hand over the left in prayer," and what diverted the command here from obligation to recommendation?

The clear indication that placing the right hand over the left in prayer is not obligatory, but rather recommended, is the hadith of the one who prayed incorrectly. In that instance, the Prophet, peace and blessings be upon him, taught him only the obligatory actions, and what he remained silent about is considered from the Sunnah. None of the predecessors (Salaf) or the Imams asserted the obligation of qabd (placing hands), and the disagreement was only between those who considered it permissible and recommended, and those who did not consider it permissible. As for Al-Shawkani's assertion of its obligation, it is refuted by the consensus that preceded him.