The ruling on the ma'mum praying on the third floor of the mosque

Question:
The questioner from outside the Kingdom says: Our area is crowded and the land is small, so we built a mosque, O Shaykh, with three floors. Is it permissible to pray on the third floor?

Answer:
Yes, it is permissible for you to pray behind the imam while you are on the third floor and the imam is on the ground floor, or vice versa, with the imam on the third floor and you on the ground floor, provided you can hear his voice.