فَإِن تَوَلَّوْا فَقُلْ حَسْبِيَ اللَّهُ لَا إِلَٰهَ إِلَّا هُوَ ۖ عَلَيْهِ تَوَكَّلْتُ ۖ وَهُوَ رَبُّ الْعَرْشِ الْعَظِيمِ (129)
(9:129) Now if they turn away from you, say to them, O Prophet, "Allah suffices me: there is no god but He: in Him have I put my trust: He is the Lord of the Great Throne."
This is the only Surah of the Quran to which Bismillah is not prefixed. Though the commentators have given different reasons for this, the correct one that which has been given by Imam Razi: namely,this is because the Holy Prophet himself did not dictate it at the beginning of the Surah. Therefore the Companions did not prefix it and their successors followed them., This is a further proof of the fact that utmost care has been taken to keep the Quran intact so that it should remain in its complete and original form.