Is it permissible to say, "There is no god but the Most Gracious (Ar-Rahman)"?

There is no doubt about the permissibility of saying "La ilaha illa al-Rahman" (There is no god but the Most Merciful), because the name "al-Rahman" is one of the beautiful names of Allah, and no one else is named with it absolutely. Al-Bukhari used this as evidence that if a disbeliever affirms the Oneness of al-Rahman, he is judged to have embraced Islam, unless it is out of stubbornness. The polytheists denied this noble name out of obstinacy and defiance. Nevertheless, it is preferable to adhere to the wordings mentioned in the Qur'an and Sunnah (definition), which is saying "La ilaha illa Allah" (There is no god but Allah).