Surah No. 2, Al Baqr, Ayat No. 21
o يَا أَيُّهَا النَّاسُ اعْبُدُواْ رَبَّكُمُ الَّذِي خَلَقَكُمْ وَالَّذِينَ مِن قَبْلِكُمْ لَعَلَّكُمْ تَتَّقُونَ
O mankind worship only your sustainer, Who has Created you and all those people before you, so that you become righteous.
Surah No. 16, An Nahl, Ayat No. 49
وَلِلّهِ يَسْجُدُ مَا فِي السَّمَاوَاتِ وَمَا فِي الأَرْضِ مِن دَآبَّةٍ وَالْمَلآئِكَةُ وَهُمْ لاَ يَسْتَكْبِرُونَ
To Allah prostrate all that, which are in the heavens and on earth, whether moving (living) creatures or the angels: and they are not arrogant (before their Lord)
o وَإِلَى رَبِّكَ فَارْغَبْ o فَإِذَا فَرَغْتَ فَانصَبْ
So when you have finished (your occupation), devote yourslef for Allah's worship. And to your Lord turn all your attention.
Surah No. 109, Al Kafirun, Ayats No. 1 to 6
o وَلَا أَنتُمْ عَابِدُونَ مَا أَعْبُدُ o لَا أَعْبُدُ مَا تَعْبُدُونَ o قُلْ يَا أَيُّهَا الْكَافِرُونَ
oلَكُمْ دِينُكُمْ وَلِيَ دِينِ o وَلَا أَنتُمْ عَابِدُونَ مَا أَعْبُدُ o وَلَا أَنَا عَابِدٌ مَّا عَبَدتُّمْ
Say : O disbelievers in Faith ! I do not worship that which you worship, Nor you will worship that which I worship. And I will not worship that which you are worshipping, Nor you will worship that which I worship.To you be your religion, and to me my religion.
Then, this surah was revealed, in which Allah has commanded the Prophet to convey to the disbelievers that for them their religion and for the believers their own. The believers worship only Allah and it was communicated to the disbelievers that if they do not want to worship Allah then it is upto them. For believers it was said that it is a must for them to worship Allah Only and must be careful not to worship those who are worshipped by the disbelievers.
The prophet Mohammad (saws) had a great importance of this surah and at most of the occassions he was reciting this surah in first rakat and surat Ikhlas in second rakat of sunnat salat of Fajr and Maghrib.
The Prophet (saws) has also guided the followers to recite this surah before going to bed in the night.
[ Ref : Tafhimul Quran Vol 6 ,tafsir of surah Al Kafirun ]