What is the ruling on saying "Bismillah" (in the name of Allah) during ablution?

Scholars differ on the ruling concerning saying "Bismillah" (in the name of Allah) before ablution (wudu'). Imam Ahmad held it to be obligatory, citing the hadith: "There is no ablution for one who does not mention the name of Allah upon it." However, the majority of scholars are of the view that it is a Sunnah (recommended practice) and not obligatory. They base their argument on the fact that the Prophet (peace be upon him) taught how to perform ablution without mentioning "Bismillah" in it, and that the Quranic verse commanding ablution does not mention "Bismillah." Furthermore, many who described the Prophet's ablution did not mention "Bismillah." The majority of scholars responded to the hadith cited by those who deem it obligatory by stating that it is weak, or that its meaning is "no complete ablution" and not "no valid ablution." Therefore, the ablution of one who does not say "Bismillah" is valid, but it is more cautious for a Muslim not to omit saying "Bismillah."