Reference: al-Muntaqaa min Fataawa Shaykh Saalih al-Fowzaan – Volume 5, Page 58, Fatwa No.87
Question: Is there any effect on the validity of the prayer whereby the imaam is praying [leading] the prayer from a raised platform to the congregation of followers?
Response: There is [some] detail [to this issue]; If the imaam was on his own, then it is permissible for him to be slightly raised such as the [height of the] stairs on the pulpit, [as] the Prophet (sal-Allaahu ‘alayhi wa sallam) ascended the pulpit and prayed (led the prayer) and then descended, so there is no harm in a slight raise so long as the imaam is on his own.
However, if there was a considerable raise then it is not permissible for the imaam to be singled out [like this] from the congregation of followers.
If however, the imaam was with [some of] the congregation of followers, then there is no harm in them being considerably raised [from the rest of the congregation of followers], so it is permissible for example that the imaam [and some of the congregation of followers] is on a higher level/floor and the rest of the congregation of followers are on lower levels/floors below him.