وَمَا مَنَعَنَا أَن نُّرْسِلَ بِالآيَاتِ إِلاَّ أَن كَذَّبَ بِهَا الأَوَّلُونَ وَآتَيْنَا ثَمُودَ النَّاقَةَ مُبْصِرَةً فَظَلَمُواْ بِهَا وَمَا نُرْسِلُ بِالآيَاتِ إِلاَّ تَخْوِيفًا
[17:59]
And nothing could have hindered Us that We should send signs except that the ancients rejected them; and We gave to Samood the she-camel-- a manifest sign-- but on her account they did injustice, and We do not send signs but to make (men) fear.
And We refrain from sending the signs, only because the men of former generations treated them as false: We sent the she-camel to the Thamud to open their eyes, but they treated her wrongfully: We only send the Signs by way of terror (and warning from evil).
Naught hindereth Us from sending portents save that the folk of old denied them. And We gave Thamud the she-camel - a clear portent save to warn.