What is the ruling on praying on a prayer rug, and the ruling on placing it in the mosque for the imam to pray on, and is that considered an innovation (bid‘ah)?

Praying on a prayer rug is permissible in principle, as the Messenger of Allah, peace and blessings be upon him, used to pray on a khumra, which is a small mat upon which one prostrates. It is also permissible for it to be larger than that. The hadith indicates the permissibility of praying on a prayer rug made of rags, palm fiber, or other materials, whether small or large, like a mat or a carpet.

However, praying on a prayer rug may be prohibited for several reasons:

1. If the prayer rug contains images of animate beings, then acquiring it is forbidden, and its images must be effaced. 2. If it has adornments and patterns that distract the worshiper and diminish their khushūʿ (devotion), then praying on it is disliked (makrūh), but the prayer is valid. 3. If one prays on the rug out of haughtiness, to avoid praying on the ground or on the general carpet in the mosque, or as a precaution against an assumed impurity. 4. If one specifically seeks to pray on a prayer rug, believing that a special rug is obligatory, whereas this was not the practice of the predecessors (the Salaf), who used to pray on the ground. 5. Designating a special rug for the Imam, even though the mosque is carpeted; this is undesirable because it might instill in his heart a sense of superiority and arrogance.

In summary, spreading a prayer rug on top of another rug is considered an innovation (bidʿah) unless there is a reason for it, such as extreme cold, hard ground, or the first rug being impure or dirty.