Is the saying true that whoever recites "There is no god but You; glory be to You! Indeed, I have been of the wrongdoers" 40 times will have their supplication answered, and is it permissible to repeat it 40 times?

The supplication of Yunus (peace be upon him) — "There is no deity except You; exalted are You. Indeed, I have been of the wrongdoers" — is a great and answered supplication, as the Prophet (peace and blessings be upon him) said: "No Muslim supplicates with it to his Lord for anything ever, except that He answers him." There is no evidence to specify repeating it forty times; the hadith reported regarding this is weak. It is permissible to repeat it without specification. The answering of supplication is contingent upon the fulfillment of its conditions and the absence of its impediments. Otherwise, Allah answers the supplication in one of three ways: expediting the answer, storing it for the Hereafter, or averting evil from the supplicant.