Quran 6:109 from Surah Al-An'aam on Divine Marvels.
وَأَقۡسَمُواْ بِٱللَّهِ جَهۡدَ أَيۡمَٰنِهِمۡ لَئِن جَآءَتۡهُمۡ ءَايَةٞ لَّيُؤۡمِنُنَّ بِهَاۚ قُلۡ إِنَّمَا ٱلۡأٓيَٰتُ عِندَ ٱللَّهِۖ وَمَا يُشۡعِرُكُمۡ أَنَّهَآ إِذَا جَآءَتۡ لَا يُؤۡمِنُونَ
English translation: And they swear by Allah strongest (of) their oaths that if came to them a sign they would surely believe in it Say Only the signs (are) with Allah And what will make you perceive that [it] when it comes not they will believe
Transliteration: wa-aqsamu bil-lahi jahda aymanihim la-in jaathum ayatun layu'minunna biha qul innama al-ayatu inda al-lahi wama yush'irukum annaha idha jaat la yu'minuna