Surah Taubah is the only Surah in the Qur’an that does not have Bismillah written at the beginning.
Reason why Surah Taubah doesn’t start with Bismillah?
Sayyidina Abdullah bin Abbas RA once asked Sayyidina Uthman RA, “Why is it that you have arranged Surah Anfal, which is from the Mathani (those Surahs that have less than a hundred verses) before Surah Bara’ah, whereas Surah Bara’ah has more than a hundred verses? You have also joined the two and have not written Bismillah between them. By joining the two, you have included them among the seven long Surahs. What is the reason for this?”
Sayyidina Uthman RA replied, “It was the practice of the Holy Prophet SAW that whenever any verses were revealed, he would instruct the scribes to include them in a certain Surah containing similar subject matter. Surah Anfal was revealed early in Madinah, while Surah Bara’ah was revealed towards the end. The Holy Prophet SAW passed away without informing us whether Surah Bara’ah was part of Surah Anfal or not. I have joined the two because their subject matter was alike and I did not write Bismillah between them (because it cannot be determined with conviction whether they are one or two Surahs). I have also included them among the seven long Surahs (because if they are one Surah, they belong with these six Surahs). [Tirmidhi]