Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates: When the Prophet (peace and blessings be upon him) recited "Bismillah al-Rahman al-Rahim (In the name of Allah, the Most Gracious, the Most Merciful)," the polytheists would mock it and say: Muhammad is mentioning the god of Yamamah, because Musaylimah (the Liar) had himself called "al-Rahman." So when this verse was revealed, the Messenger of Allah (peace and blessings be upon him) was ordered not to recite it aloud.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12193