´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)."
Related hadith on this topic
Explanation & Benefits
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