The Quran

وَقالَ الَّذينَ أوتُوا العِلمَ وَالإيمانَ لَقَد لَبِثتُم في كِتابِ اللَّهِ إِلىٰ يَومِ البَعثِ ۖ فَهٰذا يَومُ البَعثِ وَلٰكِنَّكُم كُنتُم لا تَعلَمونَ

Muhammad Asad

But those who [in their lifetime] were endowed with knowledge¹ and faith will say: "Indeed, you have been tardy in [accepting as true] what God has revealed,² [and you have waited] until the Day of Resurrection: this, then, is the Day of Resurrection: but you you were determined not to know it!"³

See surah 16, note 25.
Lit., "with regard to (fi) God's revelation (kitab)", i.e., that the dead shall be resurrected and judged by Him. It is to be noted that the verb labitha signifies "he waited [for something] or "he was tardy [with regard to something]" as well as "he stayed [in a place]" or "he remained". Evidently, in verse 55 ma labithu has the meaning of "they had not stayed" or "remained", while in verse 56 labihtum denotes "you have been tardy" or "you have waited".
Lit., "you were wont not to know" - i.e., "you persistently closed your mind to this promise".