Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Surah al-Mursalat was revealed to the Messenger of Allah (peace and blessings be upon him) while we were in a cave, and he taught me to recite it. I recite it almost exactly as he taught me, except that I do not remember with which of its two closing lines he ended the recitation: 'And when it is said to them, "Bow," they do not bow' [Surah al-Mursalat: 48] or 'Then in what statement after it will they believe?' [Surah al-Mursalat: 50]."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10117