The Quran

وَإِلىٰ ثَمودَ أَخاهُم صالِحًا ۗ قالَ يا قَومِ اعبُدُوا اللَّهَ ما لَكُم مِن إِلٰهٍ غَيرُهُ ۖ قَد جاءَتكُم بَيِّنَةٌ مِن رَبِّكُم ۖ هٰذِهِ ناقَةُ اللَّهِ لَكُم آيَةً ۖ فَذَروها تَأكُل في أَرضِ اللَّهِ ۖ وَلا تَمَسّوها بِسوءٍ فَيَأخُذَكُم عَذابٌ أَليمٌ

Muhammad Asad

AND UNTO [the tribe of] Thamud [We sent] their brother Salih¹ He said: "O my people! Worship God alone: you have no deity other than Him. Clear evidence of the truth has now come unto you from your Sustainer. "This she-camel belonging to God shall be a token for you: so leave her alone to pasture on God's earth, and do her no harm, lest grievous chastisement befall you.²

The Nabataean tribe of Thamud descended from the tribe of 'Ad mentioned in the preceding passage, and is, therefore, often referred to in pre-Islamic poetry as the "Second 'Ad". Apart from Arabian sources, "a series of older references, not of Arabian origin, confirm the historical existence of the name and people of Thamud. Thus the inscription of Sargon of the year 715 B.C. mentions the Thamud among the people of eastern and central Arabia subjected by the Assyrians. We also find the Thamudaei, Thamudenes mentioned in Aristo, Ptolemy, and Pliny" (Encyclopaedia of Islam IV, 736). At the time of which the Qur'an speaks, the Thamud were settled in the northernmost Hijaz, near the confines of Syria. Rock-inscriptions attributed to them are still extant in the region of Al-Hijr - As in the case of the 'Adite prophet Hud - and the prophet Shu'ayb spoken of in verses 85-93 of this surah - Salih is called the "brother" of the tribe because he belonged to it.
The commentators cite various legends to the effect that this she-camel was of miraculous origin. Since neither the Qur'an nor any authentic Tradition provides the least support for these legends, we must assume that they are based on the expression naqat Allah ("God's she-camel"), which has led some pious Muslims to fantastic conjectures. However, as Rashid Riza' points out (Manar VIII, 502), this expression denotes merely the fact that the animal in question was not owned by any one person, and was therefore to be protected by the whole tribe; a further, analogous expression is found in the words "God's earth" in the same verse: an illustration of the fact that everything belongs to God. The particular stress placed by Salih on good treatment of this ownerless animal - referred to in several places in the Qur'an - was obviously due to the cruel high-handedness displayed by the tribe, who, as the next two verses show, were wont to "act wickedly on earth by spreading corruption" and "gloried in their arrogance towards all who were deemed weak": in other words, their treatment of the defenceless animal was to be a "token" of their change of heart or (as is made clear in 54:27) "a test for them".