It is narrated from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: "The ablution of the one who does not say 'Bismillah' is not valid."
´It was narrated from Abu Sa'eed that:` The Messenger of Allah said: "There is no ablution for one who does not mention the Name of Allah (before doing it)."
Maulana Ataullah Sajid
Commentary: ➊ In light of this hadith, some scholars have declared reciting "Bismillah" at the beginning of ablution (wudu) to be obligatory, while other scholars have considered it to be a Sunnah. According to them, the meaning of "no ablution" is that "the ablution is not complete as it ought to be," but this interpretation is without evidence. ➋ If "Bismillah" is forgotten and remembered during the course of ablution (wudu), then it should be recited immediately; however, there is no need to repeat the ablution, because forgetfulness is excused.
Source: Commentary on Sunan Ibn Mājah by Mawlānā ‘Atā’ullāh Sājid, Page: 397