Saying Bismillah before Wudu: Essential?
Question

What is the ruling on saying Bismillah in wudu?
Praise be to Allah.Imam Ahmad ibn Hanbal’s view  on saying Bismillah before wudu
The scholars differed concerning the ruling on saying Bismillah in wudu . 
Imam Ahmad was of the view that it is obligatory, and he quoted as evidence the hadith narrated from the Prophet (peace and blessings of Allah be upon him), according to which he said: “There is no wudu for the one who does not mention the name of Allah.” (Narrated by al-Tirmidhi, 25; classed as hasan by al-Albani in Sahih al-Tirmidhi. See al-Mughni, 1/145) 
Views of the majority of scholars on saying Bismillah before wudu 
The majority of scholars – including Imam Abu Haneefah, Imam Malik and Imam al-Shafi'i – and according to one view narrated from Imam Ahmad, are of the view that saying Bismillah is one of the Sunnahs of wudu , and that it is not obligatory. 
They quoted evidence for its not being obligatory, such as: 
1 – The Prophet (peace and blessings of Allah be upon him) taught a man how to do wudu, and said to him: “Do wudu as Allah has commanded you.” (Narrated by al-Tirmidhi, 302; classed as sahih by al-Albani in Sahih al-Tirmidhi, 247). This is referring to the words of Allah (interpretation of the meaning): 
“O you who believe! When you intend to offer ss-salah (the prayer), wash your faces and your hands (forearms) up to the elbows, rub (by passing wet hands over) your heads, and (wash) your feet up to the ankles” [al-Maidah 5:6]
And there is no mention of saying Bismillah in what Allah commands here. See al-Majmu’ by al-Nawawi, 1/346 
Abu Dawud (856) narrated a more complete version of this hadith, which more clearly demonstrates that it is not obligatory to say Bismillah when doing wudu . 
It was narrated from Abu Hurayrah that the Messenger of Allah (peace and blessings of Allah be upon him) said: The prayer of any one of you is not complete unless he does wudu properly as enjoined by Allah. So he should wash his face and his hands up to the elbows, and wipe his head, and wash his feet up to the ankles…” 
The Prophet (peace and blessings of Allah be upon him) did not mention saying Bismillah, which indicates that it is not obligatory.  See al-Sunan al-Kubra by al-Bayhaqi, 1/44 
2 – Many of those who described the wudu of the Prophet (peace and blessings of Allah be upon him) did not mention that he said Bismillah; if it was obligatory it would have been mentioned. (See al-Sharh al-Mumti’, 1/130)
This is the view favoured by many of the Hanbalis such as al-Khiraqi and Ibn Qudamah. (See al-Mughni, 1/145; al-Insaf, 1/128)
Contemporary scholars’ views on saying Bismillah before wudu
Among contemporary scholars, this view was favoured by Shaykh Muhammad ibn Ibrahim and Muhammad ibn ‘Uthaymin (may Allaah have mercy on them both). (See: Fatawa al-Shaykh Muhammad ibn Ibrahim, 2/39; al-Sharh al-Mumti’, 1/130, 300) 
They responded in two ways to the hadith which is quoted by those who say that saying Bismillah is obligatory: 
1 – That the hadith is da’if (weak). 
It was classed as da’if by a number of scholars including Imam Ahmad, al-Bayhaqi, al-Nawawi and all-Bazzar. 
Imam Ahmad was asked about saying Bismillah when doing wudu , and he said: There is no sound hadith concerning that, and I do not know of any hadith that has a jayyid (good) isnad to support that. Al-Mughni, 1/145 
See al-Sunan al-Kubra by al-Bayhaqi, 1/43; al-Majmu’, 1/343; Talkhis al-Habir, 1/72 
2 – If the hadith is sahih, then what it means is “no perfect wudu” and not “no valid wudu”. See al-Majmu’, 1/347; al-Mughni, 1/146 
Based on this, then the hadith – if it is sahih – indicates that it is mustahabb to say Bismillah, but it is not obligatory. And Allah knows best. 
Based on this, if a Muslim does wudu and does not say Bismillah, then his wudu is valid, but he is missing out on the reward for doing this Sunnah. To be on the safe side, the Muslim should not omit saying Bismillah when doing wudu.