The apostle of Allah (ﷺ) died, but he did not mention to us that surah al baraah ins a part of al-anfal.

Abu Dawood said: Al-sha’bl, Abu Malik, Qatadah, and Thabit b. ‘Umarah said: The prophet( may peace be upon him) did not write”In the name of Allah, the compassionate, the merciful” until Surah al-naml was revealed. This is the meaning of what they said. Further, this is a mursal traditional(omitting the name of the companion)
Sunan Abi Dawud Book 2, Hadith 397
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Yazid al-Farisi said: I heard Ibn Abbas say: I asked Uthman ibn Affan: What moved you to put the (Surah) al-Bara'ah which belongs to the mi'in (surahs) (containing one hundred verses) and the (Surah) al-Anfal which belongs to the mathani (Surahs) in the category of as-sab'u at-tiwal (the first long surah or chapters of the Quran), and you did not write "In the name of Allah, the Compassionate, the Merciful" between them?

Uthman replied: When the verses of the Quran were revealed to the Prophet (ﷺ), he called someone to write them down for him and said to him: Put this verse in the surah in which such and such has been mentioned; and when one or two verses were revealed, he used to say similarly (regarding them). (Surah) al-Anfal is the first surah that was revealed at Medina, and (Surah) al-Bara'ah was revealed last in the Quran, and its contents were similar to those of al-Anfal. I, therefore, thought that it was a part of al-Anfal. Hence I put them in the category of as-sab'u at-tiwal (the seven lengthy surahs), and I did not write "In the name of Allah, the Compassionate, the Merciful" between them.

Sunan Abi Dawud Book 2, Hadith 396 Next Hadith → The prophet (ﷺ) did not distinguish between the two surahs until the words “In the name of Allah, the Compassionate, the merciful” was revealed to him. These are the words of Ibn al-sarh.

Sunan Abi Dawud Book 2, Hadith 398