What is the ruling on the aforementioned supplication, especially that which contains invoking the Prophet (peace be upon him) as an intercession, and is it permissible for a Muslim to supplicate with it?

It is advisable to refrain from the aforementioned supplication (du'a) because it contains two issues: The first is the repetition of the Basmalah and Istirja' 786 times without any legal (Shari'ah) evidence, which falls under the category of an added innovation (bid'ah idafiyyah). The second is seeking intercession through the Prophet, peace and blessings be upon him, which is prohibited according to the preponderant opinion. It is recommended to abandon this supplication and restrict oneself to the authentic, narrated supplications from the Prophet, peace and blessings be upon him.