Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
After Wudu, you can say this supplication: “Ashhadu an la ilaha ill-allah wahdahu la sharika lah, wa ashhadu anna Muhammadan ‘abduhu wa rasuluhu.”
In his famous work, Fiqh As-Sunnah, Sheikh Sayyid Sabiq, elaborates on the subject as follows:
`Umar reported that the Prophet (peace be upon him) 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.” (Muslim)
Abu Sa`id 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-Awsat. 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 mawquf.
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-Tirmidhi who said, “Its chain is muttarib and there is nothing authentic concerning this (supplication).”
Almighty Allah knows best.