أَلاَّ تَعْبُدُواْ إِلاَّ اللّهَ
(Saying) worship none but Allah.
This means that this Qur'an descended, perfect and detailed, with the purpose of Allah's worship alone, without any partners.
This is similar to the statement of Allah, the Exalted,
وَمَأ 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);There is no God but I, so worship Me. (21;25)
It is similar to Allah's statement,
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 (calling false deities).' (16;36)
In reference to Allah's statement,
إِنَّنِي لَكُم مِّنْهُ نَذِيرٌ وَبَشِيرٌ
Verily, I am unto you from Him a warner and a bringer of glad tidings.
This means, "Verily, I am unto you a warner of the punishment if you oppose Him (Allah), and a bringer of the good news of reward if you obey Him."
This meaning has been recorded in the authentic Hadith which states that the Messenger of Allah ascended mount As-Safa and called out to his near relatives of the Quraysh tribe. When they gathered around him, he said,
يَا مَعْشَرَ قُرَيْشٍ أَرَأَيْتُمْ لَوْ أَخْبَرْتُكُمْ أَنَّ خَيْلًا تُصَبِّحُكُمْ أَلَسْتُمْ مُصَدِّقِيَّ
O people of Quraysh, if I informed you that a cavalry was going to attack you in the morning, would you not believe me?
They replied, "We have not found you to be a liar."
He said,
فَإِنِّي نَذِيرٌ لَكُمْ بَيْنَ يَدَي عَذَابٍ شَدِيد
Verily I am a warner unto you before a severe punishment.
Concerning His statement