This name does not appear as an exact standalone noun in the Qur’an. It is instead drawn from the enumerated hadith list and from an authentic supplication of the Prophet صَلَّى اللّهُ عَليْهِ وَسَلَّم quoted below, alongside the Qur’an’s broader teaching that all timing and precedence belong to Allah.
اللَّهُمَّ اغْفِرْ لِي مَا قَدَّمْتُ وَمَا أَخَّرْتُ ... أَنْتَ الْمُقَدِّمُ وَأَنْتَ الْمُؤَخِّرُ، لَا إِلَٰهَ إِلَّا أَنْتَ
“O Allah, forgive me what I have put forward and what I have held back ... You are the Expediter and You are the Delayer, there is no god but You.”
— Sahih Muslim 771 (from the Prophet رَضِي اللّه عَنْه''s night-prayer supplication, narrated by ‘Ali ibn Abi Talib)
Al-Mu'akhkhir delays whoever and whatever He wills, with a wisdom no servant can fully perceive.