A true believer never gives up supplicating to his Lord and imploring Him for guidance. Almighty Allah teaches us to call upon Him and seek His forgiveness all the time. In the Qur’an, Almighty Allah teaches us:

“Ask Allah of His bounty. Lo! Allah is ever Knower of all things.” (An-Nisaa: 32)

Sheikh Sayyed Sabiq states in his well-known book, Fiqh us-Sunnah:

‘Umar reported that the Prophet (peace and blessings 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 servant 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” (At-Tabarani).

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.

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.