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:6:15 –

Say, "Indeed I fear, if I should disobey my Lord, the punishment of a tremendous Day."  

Say: ‘Indeed I fear, if I should rebel against my Lord, by worshipping other than Him, the chastisement of a dreadful day’, namely, the Day of Resurrection.
القرآن:٦:١٥ –

قُلْ إِنِّي أَخَافُ إِنْ عَصَيْتُ رَبِّي عَذَابَ يَوْمٍ عَظِيمٍ  

{قل إني أخاف إن عصيت ربي} بعبادة غيره {عذاب يوم عظيم} هو يوم القيامة.

See similar narrations below:

Collected by Qurʾān
quran:39:13 –

Say, "Indeed I fear, if I should disobey my Lord, the punishment of a tremendous Day."  

Say: ‘Indeed, should I disobey my Lord, I fear the chastisement of a tremendous day’.
القرآن:٣٩:١٣ –

قُلْ إِنِّي أَخَافُ إِنْ عَصَيْتُ رَبِّي عَذَابَ يَوْمٍ عَظِيمٍ  

{قل إني أخاف إن عصيت ربي عذاب يوم عظيم}.