Surah Taubah is the only Surah in the Qur’an that does not have Bismillah written at the beginning. 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]
The sequence of the Qur’an is that after Surah Fatiha appears the long Surahs viz. Surahs Baqarah, Al Imran, Nisa, Ma’idah, An’am, A’raf, Anfal and Bara’ah. These are called the “Sab’u Tiwal.” Thereafter follow those Surahs that are called the ‘Mi’ain.” These are those Surahs that consist of a little more than a hundred verses. Then, from Surah Yunus onwards up to Surah Hujurat, appear those Surahs called the “Mathani.” Thereafter, until the end of the Qur’an come those Surahs termed the “Mufassal.”
The Mufassal Surahs are further classified into three categories. The Surahs from Surah Hujurat (49) until Surah Buruj (85) are called the “Tiwal Mufassal.” Those from Surah Tariq (86) until Surah Bayyinah (98) are called the “Awsat Mufassal,” and from Surah Zilzal (99) up to Surah Nas (114) at the end are called the “Qisar Mufassal.”
BISMILLAH IS NOT RECITED AT THE BEGINNING OF SURAH TAUBAH
When a person is reciting Surah Anfal and then commences with Surah Bara’ah, he will not recite Bismillah because it cannot be conclusively determined whether the two are separate Surahs or not. It is best for the person who commences his recitation of the Qur’an from Surah Taubah that he also does not recite Bismillah. He will, of course, recite the Tawwudh (A’udhu Billahi Minash Shaytan nir Rajim). This is the opinion of Allama Jazari and Allama Shatbi .When beginning recitation of the Qur’an from anywhere in Surah Taubah (except from the beginning), then the reciter has the option to either recite Bismillah or omit to do so.
Reference : Tafsir Al Quran-Illuminating Discources