Bismillah is the First Ayah of Al-Fatihah

The Companions started the Book of Allah with Bismillah:

[بِسْمِ اللَّهِ الرَّحْمَـنِ الرَّحِيمِ ]

(1. In the Name of Allah, the Most Gracious, the Most Merciful.)
The scholars also agree that Bismillah is a part of an Ayah in Surat An-Naml (chapter 27). They disagree over whether it is a separate Ayah before every Surah, or if it is an Ayah, or a part of an Ayah, included in every Surah where the Bismillah appears in its beginning. Ad-Daraqutni also recorded a Hadith from Abu Hurayrah from the Prophet that supports this Hadith by Ibn Khuzaymah. Also, similar statements were attributed to `Ali, Ibn `Abbas and others.
The opinion that Bismillah is an Ayah of every Surah, except Al-Bara'ah (chapter 9), was attributed to (the Companions) Ibn `Abbas, Ibn `Umar, Ibn Az-Zubayr, Abu Hurayrah and `Ali. This opinion was also attributed to the Tabi`in: `Ata', Tawus, Sa`id bin Jubayr, Makhul and Az-Zuhri. This is also the view of `Abdullah bin Al-Mubarak, Ash-Shafi`i, Ahmad bin Hanbal, (in one report from him) Ishaq bin Rahwayh and Abu `Ubayd Al-Qasim bin Salam. On the other hand, Malik, Abu Hanifah and their followers said that Bismillah is not an Ayah in Al-Fatihah or any other Surah. Dawud said that it is a separate Ayah in the beginning of every Surah, not part of the Surah itself, and this opinion was also attributed to Ahmad bin Hanbal.