The Quran

وَما مَنَعَنا أَن نُرسِلَ بِالآياتِ إِلّا أَن كَذَّبَ بِهَا الأَوَّلونَ ۚ وَآتَينا ثَمودَ النّاقَةَ مُبصِرَةً فَظَلَموا بِها ۚ وَما نُرسِلُ بِالآياتِ إِلّا تَخويفًا

Muhammad Asad

And nothing has prevented Us from sending [this message, like the earlier ones,] with miraculous signs [in its wake], save [Our knowledge] that the people of olden times [only too often] gave the lie to them:¹ thus, We provided for [the tribe of] Thamud the she-camel as a light-giving portent, and they sinned against it.² And never did We send those signs for any other purpose than to convey a warning.

This highly elliptic sentence has a fundamental bearing on the purport of the Qur'an as a whole. In many places the Qur'an stresses the fact that the Prophet Muhammad, despite his being the last and greatest of God's apostles, was not empowered to perform miracles similar to those with which the earlier prophets are said to have reinforced their verbal messages. His only miracle was and is the Qur'an itself - a message perfect in its lucidity and ethical comprehensiveness, destined for all times and all stages of human development, addressed not merely to the feelings but also to the minds of men, open to everyone, whatever his race or social environment, and bound to remain unchanged forever. Since the earlier prophets invariably appealed to their own community and their own time alone, their teachings were, of necessity, circumscribed by the social and intellectual conditions of that particular community and time; and since the people to whom they addressed themselves had not yet reached the stage of independent thinking, those prophets stood in need of symbolic portents or miracles (see surah 6, note 94) in order to make the people concerned realize the inner truth of their mission. The message of the Qur'an, on the other hand, was revealed at a time when mankind (and, in particular, that part of it which inhabited the regions marked by the earlier, Judaeo-Christian religious development) had reached a degree of maturity which henceforth enabled it to grasp an ideology as such without the aid of those persuasive portents and miraculous demonstrations which in the past, as the above verse points out, only too often gave rise to new, grave misconceptions.
See the second paragraph of 7:73 and the corresponding note 57. Although there is absolutely no indication in the Qur'an that the she-camel referred to was of miraculous origin, it was meant to be a test for the people of Thamud (cf. 54:27), and thus a "light-giving portent" (mubsirah).