Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.034.

‘Umar reported that the Prophet, upon whom be peace, said, “If one completes (and perfects) the ablution and then says, ‘I testify that there is no god except Allah, the One Who has no partner, and that Muhammad is His slave and Messenger,” the eight gates of paradise will be opened for him and he may enter any of them that he wishes.” (Related by Muslim.)

Abu Sa’eed al-Khudri reported that the Prophet said, “Whoever makes ablution and says, ‘Glory be to Thee, O Allah, and the praise be to Thee. I bear witness that there is no god except You. I beg Your forgiveness and I repent unto you,’ will have it written for him, and placed on a tablet which will not be broken until the Day of Resurrection. This hadith is related by at-Tabarani in al-Ausat. Its narrators are of the sahih. An-Nasa’i has it with the wording. “It will be stamped with a seal, placed below the throne, and it will not be broken until the Day of Resurrection.” The correct statement is that it is mauqoof.

As for the supplication, “Allah, cause me to be from among the repentant, and cause me to be from among the pure,” it has been narrated by at-Tirmizhi who said, “Its chain is muzhtarib and there is nothing authentic concerning this (supplication).”