Is it forbidden to clasp the right hand with the left hand when establishing prayer, because people used to hide idols behind the Messenger of Allah, peace and blessings be upon him?

Holding the right hand with the left before prayer is not legislatively required. If the questioner means not raising the hands when saying the opening takbir (Allahu Akbar), then the answer is that raising the hands when saying the opening takbir is a Sunnah. Some scholars have mentioned that the wisdom behind this is to differ from the hypocrites who used to fold their arms to conceal their idols, or to signify the rejection of worldly life. However, the verified position is that raising the hands is a Sunnah from the Prophet, peace and blessings be upon him.