In which Surah is the verse that the Prophet (peace be upon him) used to recite, and then follow it with the supplication: "O Allah, grant my soul its piety and purify it, for You are the best of those who purify it. You are its Guardian and Master"?

The verse at which the Prophet, peace and blessings be upon him, used to supplicate with this prayer is: (And [by] the soul and Him who proportioned it And inspired it [with discernment of] its wickedness and its righteousness). And his supplication was: "O Allah, grant my soul its piety, purify it, for You are the best of those who purify it. You are its Guardian and its Master."