Is the formula for the Basmalah, when commencing any act, "Bismillāh" only, or "Bismillāh al-Raḥmān al-Raḥīm"?

The Sunnah, when commencing any lawful act, is to say "Bismillah" (In the name of Allah). It is preferable to add "Ar-Rahman Ar-Rahim" (The Most Gracious, the Most Merciful) to it, based on the Prophet's (peace be upon him) saying: "Every important matter that is not begun with 'Bismillah Ar-Rahman Ar-Rahim' is cut off [lacking]." The Prophet (peace be upon him) began his letters with it, Allah began His Book with it, and scholars began their books with it. Exceptions to this include certain situations, such as saying "Bismillah" when slaughtering an animal, where scholars have differed on the desirability of adding "Ar-Rahman Ar-Rahim."