3:9
رَبَّنَآ إِنَّكَ جَامِعُ ٱلنَّاسِ لِيَوْمٍۢ لَّا رَيْبَ فِيهِ ۚ إِنَّ ٱللَّهَ لَا يُخْلِفُ ٱلْمِيعَادَ
Our Lord, surely You will gather the people for a Day about which there is no doubt. Indeed, Allah does not fail in His promise."
Translation · Saheeh International
Tafsir
Ibn Kathīr (abridged)
The Mutashabihat and Muhkamat Ayat
Allah states that in the Qur'an, there are Ayat that are Muhkamat, entirely clear and plain, and these are the foundations of the Book which are plain for everyone. And there are Ayat in the Qur'an that are Mutashabihat not entirely clear for many, or some people. So those who refer to the Muhkam Ayat to understand the Mutashabih Ayat, will have acquired the correct guidance, and vice versa. This is why Allah said,
هُنَّ أُمُّ الْكِتَـبِ
(They are the foundations of the Book), meaning, they are the basis of the Qur'an, and should be referred to for clarification, when warranted,
وَأُخَرُ مُتَشَـبِهَـتٌ
(And others not entirely clear) as they have several meanings, some that agree with the Muhkam and some that carry other literal indications, although these meaning might not be desired.