7:73  And to Thamud their brother Salih.  He said: “O my people! worship Allah, you have no deity other than Him.  Clear proof has already come to you from your Rabb.  This is Allah’s she-camel— for you a Sign; so leave her alone to pasture in the earth of Allah, and do not harm her with any harm, lest a painful punishment overtake you.

7:73 The tribe of Thamūd might have descended from the tribe of ‘Ād, therefore, they are often referred to as the second ‘Ād (7:65,74; 11:61-68), although geographically they came to inhabit a territory in the northernmost Hijāz, covering the land of al-Hijr (15:80), which is in the plains of Wādi al-Qura in the peninsula between Arabia and Syria. As seen with Hūd (v. 65) and Shu’aib (v. 85), Sālih was also addressed as brother, indicating he was a familiar member of a well-knit community.
                There is nothing miraculous or divine about the she-camel, although she is mentioned as naqat Allāh (lit. Allāh’s she-camel, 91:13). Prophet Sālih placed particular stress on humane treatment towards the ownerless she-camel as an eye opener for a community that was cruel towards all those who were weak, even if it were an animal. The ownerless she-camel was made a sign for the Thamūd in the sense that the people were instructed to leave her alone to pasture in the earth of Allāh, and not to harm her in any way (11:64; 91:13). Thus, it was a test for the Thamūd whether they would act arrogantly or obey the divine command to restrain their cruelty.