Allah said,

[وَإِلَى ثَمُودَ]

(And to Thamud), meaning, to the tribe of Thamud, We sent their brother Salih,

[قَالَ يَاقَوْمِ اعْبُدُواْ اللَّهَ مَا لَكُمْ مِّنْ إِلَـهٍ غَيْرُهُ]

(He said: "O my people! Worship Allah! You have no other god but Him.'') All Allah's Messengers called to the worship of Allah alone without partners. Allah said in other Ayat,

[وَمَآ أَرْسَلْنَا مِن قَبْلِكَ مِن رَّسُولٍ إِلاَّ نُوحِى إِلَيْهِ أَنَّهُ لا إِلَـهَ إِلاَّ أَنَاْ فَاعْبُدُونِ ]

(And We did not send any Messenger before you but We revealed to him (saying): "None has the right to be worshipped but I, so worship Me.'') [21:25] and,

[وَلَقَدْ بَعَثْنَا فِى كُلِّ أُمَّةٍ رَّسُولاً أَنِ اعْبُدُواْ اللَّهَ وَاجْتَنِبُواْ الْطَّـغُوتَ]

(And verily, We have sent among every Ummah a Messenger (proclaiming): "Worship Allah (Alone), and avoid Taghut (all false deities)'') [16:36].