What is the ruling regarding someone whose tongue slipped while reciting Surat Al-Falaq during prayer, uttering a wrong word, knowing that he suffers from obsessive whispers of disbelief?

There is nothing upon you, for Allah has forgiven errors, forgetfulness, and what one is forced to do, due to His saying, the Exalted: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended," and due to His saying, the Exalted: "Our Lord, do not impose blame upon us if we have forgotten or erred." And Allah said in response to it: "I have done so." So, ignore the whisperings (waswas); for indulging in them leads to great evil, and these whisperings will not harm you, by Allah's permission.