Is the supplication "Kanz al-ʿArsh" (Treasure of the Throne) that is mentioned known, and what is its virtue?

This supplication is unknown in the Sunnah and is thought to be an invention by some Sufis as part of the "awrad" (regular litanies) they prescribe for their disciples. It is not permissible to follow them in this, because supplication is an act of worship based on divine revelation and adherence (to what is prescribed), not on innovation.

Ibn Taymiyyah said: "Remembrances (dhikr) and supplications (du'a) are among the best acts of worship, and acts of worship are based on divine revelation and adherence, not on whims and innovation." He added that the Prophetic supplications and remembrances are the best, and anything else may be forbidden, disliked, or involve shirk. He concluded that it is not permissible for anyone to institute for people a type of remembrance that is not legislated (by the Sharia) and make it a regular act of worship, for that is an innovation in religion.