وَيَقُولُونَ مَتَىٰ هَـٰذَا ٱلْوَعْدُ إِن كُنتُمْ صَـٰدِقِينَ 
  Wayaqooloona mat a  h atha  alwaAAdu in kuntum  sa diqeen a      And yet, they [who deny the truth] are wont to ask, "When is that    promise [of resurrection and judgment] to be fulfilled? [Answer    this, O you who believe in it,] if you are men of truth!"
   - Mohammad Asad
     
They ask: "When will this promise be fulfilled, tell us if what    you say be true?"
   - Muhammad Farooq-i-Azam Malik
   They ask 'the believers', 'When will this threat come to pass if what you say is true?'
   - Mustafa Khattab
     
And they say: When will this promise be fulfilled, if ye are    truthful?
   - Marmaduke Pickthall
   They say: "When will this promise come to pass if ye speak the    truth?"
   - Abdullah Yusuf Ali