Surat Al-'Isrā' - سورة الإسراء

وَإِذَا قَرَأْتَ الْقُرْآنَ جَعَلْنَا بَيْنَكَ وَبَيْنَ الَّذِينَ لَا يُؤْمِنُونَ بِالْآخِرَةِ حِجَابًا مَّسْتُورًا

Muhsin Khan: And when you (Muhammad SAW) recite the Quran, We put between you and those who believe not in the Hereafter, an invisible veil (or screen their hearts, so they hear or understand it not).