15. God hath granted you leave to prostrate yourselves on any surface that is clean, for We have removed in this regard the limitation that had been laid down in the Book # 10The requirements of prayer in previous Dispensations have often included prostration. In the Arabic Bayan the Bab called upon the believers to lay their foreheads on surfaces of crystal when prostrating. Similarly, in Islam, certain restrictions are imposed with regard to the surface on which Muslims are permitted to prostrate. Baha'u'llah abrogates such restrictions and simply specifies "any surface that is clean".
2. In Islam it is forbidden to perform prostrations on certain things even though they are
ceremonially pure. "The Book" in this case is presumably the Qur'an.