Why There Is No Bismillah in Surah At-Tawbah
Question

Do we have to say Bismillah when reading Surah At-Tawbah from the middle? Also why is there no Bismillah in the beginning of Surah Tawbah?
Praise be to Allah.Reciting the Basmalah before reading the Quran
Part of the etiquette of reciting the Quran is for the reciter to always say Bismillah ir-Rahman ir-Rahim (In the name of Allah, the Most Gracious, the Most Merciful) at the beginning of every Surah, except Surah At-Tawbah.
Imam An-Nawawi (may Allah have mercy on him) said: 
“He should always recite Bismillah ir-Rahman ir-Rahim at the beginning of every Surah except Surah Bara’ah (At-Tawbah), because the majority of scholars said that it is a verse wherever it is written in the Mushaf, and it was written at the beginning of all the Surahs except Surah At-Tawbah. If he recites it at the beginning of every Surah (other than At-Tawbah), then there is certainty that he had recited the entire Quran or Surah, and if he fails to recite the Basmalah, he has omitted part of the Quran according to the majority.” (At-Tibyan fi Adab Hamalat Al-Quran, 100.)
But if he recites from partway through a Surah, he does not have to recite the Basmalah and it is sufficient for him to seek refuge with Allah (by saying A‘udhu Billahi min ash-Shaytan ir-rijim (I seek refuge with Allah from the accursed devil), but if he does say the Basmalah there is nothing wrong with that. 
Why doesn’t Surah At-Tawbah start with Bislmillah?
The reason why the Basmalah does not appear at the beginning of Surah At-Tawbah is that the Companions (may Allah be pleased with them) did not write it at the beginning of this Surah in the Mushaf, following the example of the caliph `Uthman (may Allah be pleased with him). At-Tirmidhi narrated in As-Sunan that Ibn `Abbas (may Allah be pleased with him) said: “I said to `Uthman ibn `Affan: ‘What made you put Al-Anfal which is one of the Mathani, next to Bara’ah [At-Tawbah], which is one of the Mi’in? Why did you not put the line Bismillah ir-Rahman ir-Rahim in between them when you put it at the beginning of the rest of Al-Sab’ Al-Tiwal [the long seven Surahs]?” `Uthman said: “The Messenger of Allah (peace and blessings of Allah be upon him) used to receive revelation of Surahs with many verses. When they were revealed, he would call his scribes and tell them, ‘Put these verses in the Surah where such-and-such is mentioned.’ Al-Anfal was one of the first Surahs to be revealed in Madinah, and Bara’ah (At-Tawbah) was one of the last parts of the Quran to be revealed. Its stories were similar to the stories mentioned in Al-Anfal, so it was thought that it was part of it. The Messenger of Allah (peace and blessings of Allah be upon him) was taken [i.e., died] without explaining whether it was indeed part of it, so they were put next to one another, and the line Bismillahi ir-Rahman ir-Rahim was not written between them, and it [At-Tawbah] was put among the Sab’ Al-Tiwal [seven long Surahs].” (Fatawa Al-Lajnah Ad-Da’imah, 4/225)
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said: … The correct opinion is that there is no Basmalah between it (Surah At-Tawbah) and Al-Anfal, because the Basmalah is a verse of the Book of Allah, may He be Glorified and Exalted, and if the Messenger (blessings and peace of Allah be upon him) did not say to put the Basmalah between the two Surahs, they did not put it between them. The Prophet (blessings and peace of Allah be upon him) is the one who decided and said: Put the Basmalah, and he did not tell them to put the Basmalah between Surah Al-Anfal and Surah At-Tawbah, so they did not write it. But it remains to be said: if he did not give instructions to this effect, why is there a separation between it and Surah Al-Anfal? Why did they not make it one Surah? We say: Yes, they did not make it one Surah because they were uncertain as to whether it was the same Surah as Al-Anfal or they were two separate Surahs, so they said: Let us make a separation between the two Surahs and not write the Basmalah. This is the correct opinion concerning the absence of the Basmalah between Surah Al-Anfal and Surah At-Tawbah.
End quote from Liqa’ Al-Bab Al-Maftuh, no. 18
And Allah knows best.