وَيَسْتَعْجِلُونَكَ بِٱلْعَذَابِ ۚ وَلَوْلَآ أَجَلٌ مُّسَمًّى لَّجَآءَهُمُ ٱلْعَذَابُ وَلَيَأْتِيَنَّهُم بَغْتَةً وَهُمْ لَا يَشْعُرُونَ
WayastaAAjiloonaka bi a lAAa tha bi walawl a ajalun musamman laj a ahumu alAAa tha bu walayatiyannahum baghtatan wahum l a yashAAuroon a Now they challenge thee to hasten the coming upon them of [God's] chastisement:51 and indeed, had not a term been set [for it by God], that suffering would already have come upon them! But indeed, it will most certainly come upon them of a sudden, and they will be taken unawares.
 - Mohammad Asad
They challenge you to hasten the scourge on them. Had a time not been fixed for it, the scourge would already have overtaken them. It is going to come suddenly and catch them unaware.
 - Muhammad Farooq-i-Azam Malik
They challenge you 'O Prophet' to hasten the punishment. Had it not been for a time already set, the punishment would have certainly come to them 'at once'. But it will definitely take them by surprise when they least expect it.
 - Mustafa Khattab
They bid thee hasten on the doom (of Allah). And if a term had not been appointed, the doom would assuredly have come unto them (ere now). And verily it will come upon them suddenly when they perceive not.
 - Marmaduke Pickthall
They ask thee to hasten on the Punishment (for them): had it not been for a term (of respite) appointed the Punishment would certainly have come to them: and it will certainly reach them of a sudden while they perceive not! 3485
 - Abdullah Yusuf Ali