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:2:88 –

And they said, "Our hearts are wrapped." But, ˹in fact˺, Allah has cursed them for their disbelief, so little is it that they believe.  

And they say, to the Prophet mockingly: ‘Our hearts are encased’ (ghulf is the plural of aghlaf), that is to say, wrapped up in covers and cannot comprehend what you say; God, exalted be He, says: Nay (bal introduces the rebuttal), but God has cursed them, removed them far from His mercy and degraded them when they rejected ˹the messengers˺, for their unbelief, which is not the result of anything defective in their hearts; and little will they believe (fa-qalīlan mā yu’minūn: the mā here is extra, emphasising the ‘littleness’ involved): that is, their belief is minimal.
القرآن:٢:٨٨ –

وَقَالُوا قُلُوبُنَا غُلْفٌ ۚ بَلْ لَعَنَهُمُ اللَّهُ بِكُفْرِهِمْ فَقَلِيلًا مَا يُؤْمِنُونَ  

{وقالوا} للنبي استهزاء {قلوبنا غُلْفٌ} جمع أغلف أي مغشاة بأغطية فلا تعي ما تقول قال تعالى: {بل} للإضراب {لعنهم الله} أبعدهم من رحمته وخذلهم عن القبول {بكفرهم} وليس عدم قبولهم لخلل في قلوبهم {فقليلا ما يؤمنون} ما زائدة لتأكيد القلة أي: إيمانهم قليل جداً.