Is it sufficient to say "Bismillah" when performing ablution, or is it obligatory to say "Bismillah Ar-Rahman Ar-Rahim"?

Saying "Bismillah" is prescribed at the beginning of ablution (wudu'). It is sufficient to say: "Bismillah (In the Name of Allah)," and if one says: "Bismillahir-Rahmanir-Rahim (In the Name of Allah, the Most Gracious, the Most Merciful)," it is more complete according to some scholars.