When we start the Wudu would we say only 'Bismillah' or 'Bismillahi Rahmani al-Raheem?'

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.

Imam An-Nawawi said: “The perfect way is to recite the complete verse of Bismillaahi ar-Rahmaani ar-Raheem; if one recites only Bismillah, he will get the virtues of mentioning Allah's Name.

Many scholars such as Al-Mawardi, Imam Al-Haramayn (i.e. the Imam of the Two Custodian Mosques), Ibn As-Sabbaagh and others mentioned this ruling clearly in their books and Fataawa. This is the opinion of the Shaafi'i School of jurisprudence and one of the sayings in the Maaliki School of jurisprudence.

For more benefit on the detailed description of ablution, please refer to Fataawa 82845 and 83164.

Allah knows best.