Doubt concerning the Cleanliness. of the Place of Prayer
Question :
When any of us moves from one apartment to another bearing in mind that all of the apartments are carpeted - is it permissible for us to pray on them, since we don't know whether the previous occupants were Muslims or not?
Answer:
Essentially, all things are considered pure, so nothing may be ruled as impure, unless there is some evidence to show that a particular thing is unclean, and this proven uncleanness is present in this place. If these two things are not proven, then the Muslim may pray, and his prayer will be correct.
And may peace and blessings be upon our Prophet, Muhammad and upon his family and Companions.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 2 Pages 30-31