Saying Bismi Allah in the Middle of Ablution If One Forgets to Say it at the Beginning
4-8-2021 | IslamWeb
Question:
Can a person say bismillah in the middle of wudu,if he fails to say at the start,according to the opinion of some scholars that it is a must?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
First of all, we would like to say that mentioning the Name of Allah in ablution is a Sunnah and is not obligatory according to the view of the majority of the scholars. Nonetheless, the Hanbali School of jurisprudence is of the view that it is obligatory, and that it is waived if one forgets to say it, and that whoever forgets it at the beginning of ablution and remembers during it, then he says Bismi Allah (i.e. in the Name of Allah) and continues his ablution and does not repeat the ablution from its beginning.
Al-Bahooti 
from the Hanbali School of jurisprudence, said in Kash-Shaf Al-Qinaa’:
“If he remembers it (i.e. saying Bismi Allah) in the course of doing it (i.e. during what is mentioned whether it is ablution or Ghusl [ritual bath] or Tayammum (then he says Bismi Allah and continues), because when it was pardoned because of forgetfulness about the entirety of purity, then if it was forgotten only in part of it then it is also pardoned with greater reason. The author of Hashiyat at-Tanqeeh said in the footnote: ‘This is the adopted view of the Hanbali School and it is also the view adopted by the majority of the scholars of the School [Hanbali].” [End of quote]
Therefore, there is nothing wrong with the one who forgets to say the Bismillah (i.e. saying Bismi Allah) at the beginning of ablution to say it during it, and if he leaves it, then he is not required to do anything, because it is only desirable according to the opinion of the majority of the scholars, as we have already mentioned, contrarily to the view of the Hanbali School.
For more benefit on the Wajib (obligatory) and Sunnah acts of ablution and Prayer, please refer to Fatwa 84841.
Allah knows best.