Fabi ayyi aalaaa'i Rabbikumaa tukazzibaan

فَبِأَىِّ ءَالَآءِ رَبِّكُمَا تُكَذِّبَانِ

Word By Word:

Fabi ayyi: So which

alaa'i: (of the) favours

Rabbikuma: (of) your Lord

tukazzibaan: will ye both deny?

Translation:

Then which of the favors of your Lord will ye deny?

Ayah Tafseer:

Obviously, the person who is a denier of Paradise and its rewards, actually denies many of Allah Almighty's praiseworthy attributes. Even if he believes in Allah, he holds a bad opinion about Him. In his view, He is a perverse ruler in whose lawless kingdom doing good is an exercise in futility. He is either blind or deaf and is, therefore, wholly unaware as to who in his kingdom is making sacrifices of life, wealth and labor for His sake. He is un-appreciative and cannot distinguish between good and evil. Or as he seems to think, He is helpless and powerless. He may be very appreciative of the good but is unable to reward the doer of it. That is why it has been said: When in the Hereafter good will be rewarded with good in front of your very eyes, will you even then disavow the praiseworthy attributes of your Lord.

Source: Yusuf Ali Translations and Tafheemul Quran by Abul A'la Maududi