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:51:5 –

Indeed, what you are promised is true.  

assuredly what you are promised (mā, ‘what’, relates to the verbal action), in other words, the promise given to them of resurrection and other matters, is true, is indeed a true promise,
القرآن:٥١:٥ –

إِنَّمَا تُوعَدُونَ لَصَادِقٌ  

{إنما توعدون} ما مصدرية، أي وعدهم بالبعث وغيره {لصادق} لوعد صادق.

See similar narrations below:

Collected by Qurʾān
quran:77:7 –

Indeed, what you are promised is to occur.  

Surely that which you are promised, O disbelievers of Mecca, in the way of resurrection and chastisement, will befall: it will be, without any doubt.
القرآن:٧٧:٧ –

إِنَّمَا تُوعَدُونَ لَوَاقِعٌ  

{إنما توعدون} أي يا كفار مكة من البعث والعذاب {لواقع} كائن لا محالة.