What is the ruling on a worshiper's (ma'mum) agreement with the imam regarding saddl (letting the hands hang down) or qabd (grasping the hands), and is he sinful for disagreeing with him on how to place the hands?

Clasping in prayer is a confirmed Sunnah from the Prophet, peace and blessings be upon him. Therefore, the one praying, whether an imam or a follower, should act upon it. If the imam does not clasp, the follower should clasp, and this is not considered a violation of following the imam. The majority of scholars view clasping as a Sunnah, while the well-known position of the Maliki school is that it is not a Sunnah.