Supplication During Ablution
There is no proof that the Prophet (peace and blessings of Allah be upon him) said any du’a during wudu whilst washing or wiping his limbs, etc. Any such du’as that are known are bid’ah (innovation) and have no basis. What is known from Islamic sources is that one should say “Bismillah il-Rahman il-Raheem” at the beginning, and after finishing one should recite the Shahadatayn (testimony of faith) and say, “Allahumma aj’alni min al-tawwabeen waj’alni min al-mutatahhireen (O Allah, make me one of those who repent and make me one of those who purify themselves).”
And Allah knows best.