What is the ruling on the prayer of one who prayed on a prayer rug underneath which was a mat defiled with urine, while he knew of its impurity and thought his prayer was permissible?

Performing prayer on a prayer rug beneath which is a ritually impure mat is valid, though disliked, according to the Hanbalis and Shafi'is. The Malikis hold that there is no harm in a sick person praying on a bed.