His statement, “not one of the limbs used in prostration,” means that it is not permissible to prostrate upon a barrier consisting of one of the limbs used in prostration—for example, by placing one’s forehead upon one’s palms, placing one’s hands one upon the other, or placing one’s feet one upon the other. If he does so, it is as though he has prostrated upon a single limb.

His statement, “even with a barrier that is not one of the limbs used in prostration”: he did not explain the ruling on prostrating upon a barrier when it is not one of the limbs used in prostration. Rather, he only explained that prostration is valid with such a barrier. So what is the ruling on placing a barrier?

The scholars said: barriers are of two types:

The first type is that which is connected to the worshipper. It is disliked to prostrate upon this unless there is a need, such as a garment one is wearing, a worn outer cloak, a headcloth, and the like. The evidence for this is the hadith of Anas ibn Malik, may Allah be pleased with him, who said: “We used to pray with the Prophet, peace and blessings be upon him, in intense heat. If one of us was unable to place his forehead firmly upon the ground, he would spread out his garment and prostrate upon it.” (1)

His words, “If one of us was unable,” indicate that they did not do this when they were able to prostrate directly upon the ground. Moreover, the expression, “If one was unable,” indicates that it is disliked and should only be done when there is a need.

The second type is that which is separate from the worshipper. There is nothing wrong with it, nor is it disliked.