Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
quran:36:9 –

And We have put before them a barrier and behind them a barrier and covered them, so they do not see.  

And We have set before them a barrier (read saddan or suddan in both instances) and behind them a barrier; so We have covered them, so they do not see — this is also figurative, depicting the way in which the paths of faith are closed to them.
القرآن:٣٦:٩ –

وَجَعَلْنَا مِنْ بَيْنِ أَيْدِيهِمْ سَدًّا وَمِنْ خَلْفِهِمْ سَدًّا فَأَغْشَيْنَاهُمْ فَهُمْ لَا يُبْصِرُونَ  

{وجعلنا من بين أيديهم سدا ومن خلفهم سدا} بفتح السين وضمها في الموضعين {فأغشيناهم فهم لا يبصرون} تمثيل أيضا لسدّ طرق الإيمان عليهم.