The Quran

وَإِمّا نُرِيَنَّكَ بَعضَ الَّذي نَعِدُهُم أَو نَتَوَفَّيَنَّكَ فَإِلَينا مَرجِعُهُم ثُمَّ اللَّهُ شَهيدٌ عَلىٰ ما يَفعَلونَ

Muhammad Asad

And whether We show thee [in this world] something of what We hold in store for those [deniers of the truth],¹ or whether We cause thee to die [before that retribution takes place - know that, in the end], it is unto Us that they must return; and God is witness to all that they do.²

Lit., "of what We promise them" or "of what We threaten them with" - i.e., the inevitable retribution, sometimes even in this world, which a deliberate denial of the truth brings in its wake.
The above verse is addressed, in the first instance, to the Prophet, and relates to those of his contemporaries who refused to acknowledge the truth of the Qur'anic revelation. In its wider sense, however, it is addressed to every believer who might find it incomprehensible that life-long suffering is often the lot of the righteous, while many wrongdoers and deniers of the truth apparently remain unscathed and are allowed to enjoy the good things of life. The Qur'an solves this apparent paradox by making it clear that, in comparison with the life to come, the life in this world is but a brief moment, and that it is only in the hereafter that man's destiny reveals itself in all its true aspects. Cf. 3:185 - "only on the Day of Resurrection will you be requited in full [for whatever you have done] ... for the life of this world is nothing but an enjoyment of self-delusion".