Is saying "Bismillah Ar-Rahman Ar-Rahim" (In the Name of Allah, the Most Gracious, the Most Merciful) when performing ablution sufficient, or is only "Bismillah" (In the Name of Allah) sufficient?

The naming (mentioning God's name) during ablution is done by saying "Bismillah" (In the name of Allah). Scholars have differed regarding adding "Ar-Rahman Ar-Rahim" (The Most Gracious, the Most Merciful) to it. Some believe that adding it makes it more complete, while others prefer to suffice with "Bismillah" only. However, no one has ever said that adding to it invalidates the ablution.