How sound is the statement that publishing (There is no god but You; glory be to You, indeed I have been of the wrongdoers) earns its publisher thirty thousand good deeds?

There is no textual religious evidence that specifies a certain number of good deeds (hasanat) for reciting "Lā ilāha illā Anta, Subḥānaka innī kuntu minaẓ-ẓālimīn (There is no deity except You; exalted are You. Indeed, I have been of the wrongdoers)." Such a specification depends on revelation, and it has not been mentioned in the Qur'an or authentic Sunnah. However, great virtues have been reported for this remembrance, as the Messenger of Allah, peace and blessings be upon him, said: "The supplication of Dhul-Nun (Jonah) when he supplicated while he was in the belly of the whale: 'There is no deity except You; exalted are You. Indeed, I have been of the wrongdoers.' For indeed, no Muslim man ever supplicates with it for anything but Allah responds to him."