Surah Al-An'am (The Cattle), Verse 109

وَاَقْسَمُوْا بِاللّٰهِ جَهْدَ اَیْمَانِهِمْ لَىِٕنْ جَآءَتْهُمْ اٰیَةٌ لَّیُؤْمِنُنَّ بِهَا قُلْ اِنَّمَا الْاٰیٰتُ عِنْدَ اللّٰهِ وَمَا یُشْعِرُكُمْ اَنَّهَاۤ اِذَا جَآءَتْ لَا یُؤْمِنُوْنَ ۟

Wa aqsamoo billaahi jahda aimaanihim la'in jaaa'at hum Aayatul la yu'minunna bihaa; qul innamal Aayaatu 'indal laahi wa maa yush'irukum annahaaa izaa jaaa'at laa yu'minoon

“They swear their strongest oaths by Allah, that if a (special) sign came to them, by it they would believe. Say: "Certainly (all) signs are in the power of Allah: but what will make you (Muslims) realise that (even) if (special) signs came, they will not believe."”

Quran 6:109 (Yusuf Ali Translation)