What is the ruling on sitting in the mosque on a chair for someone who is strong, or leaning on a cushion with a backrest?

There is no objection to sitting in the mosque on a chair or anything else for the weak and others, based on the Almighty's saying: "those who remember Allah while standing or sitting or on their sides." This sitting is general and includes sitting directly on the mosque's carpet or on a chair. Sitting on the aforementioned device does not fall under the definition of innovation (bid'ah), because innovation is an invented method in religion that rivals the Sharia, whereas this device is merely an ordinary invention useful in matters of customs, and is not a form of worship.

Therefore, there is no objection to sitting on a chair in the mosque, whether there is a need or not. This ruling applies to sitting in the mosque outside of prayer. As for during prayer, it is not permissible to pray on a chair for one who is able to stand if the prayer is obligatory, but it is permissible for supererogatory (nafl) prayers.