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:11:26 –

That you not worship except Allah. Indeed, I fear for you the punishment of a painful day."  

That, you worship none but God. Lo! I fear for you, should you worship other than Him, the chastisement of a painful day’, painful in this world and in the Hereafter.
القرآن:١١:٢٦ –

أَنْ لَا تَعْبُدُوا إِلَّا اللَّهَ ۖ إِنِّي أَخَافُ عَلَيْكُمْ عَذَابَ يَوْمٍ أَلِيمٍ  

{أن} أي بأن {لا تعبدوا إلا الله إني أخاف عليكم} إن عبتم غيره {عذاب يوم أليم} مؤلم في الدنيا والآخرة.