بُلُوغُ الْمَرَامِ
Bulugh al-Maram 282
The Book of Prayer
وَعَنْ أَبِي هُرَيْرَةَ - رضى الله عنه - قَالَ : قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ إِذَا قَرَأْتُمْ اَلْفَاتِحَةِ فَاقْرَءُوا : ( بِسْمِ اَللَّهِ اَلرَّحْمَنِ اَلرَّحِيمِ ) , فَإِنَّهَا إِحْدَى آيَاتِهَا } رَوَاهُ اَلدَّارَقُطْنِيُّ , وَصَوَّبَ وَقْفَهُ . [1]
Narrated Abu Hurairah (RA): Allah's Messenger (ﷺ) said: "Whenever you recite Surat Al-Fatiha, recite Bismillahir-Rahmanir-Rahim, for it is one of its Ayat (Verses)." [Reported by ad-Daraqutni who verified it as Mawquf - the saying of Abu Hurairah (RA)].
Sunnah.com reference: Book 2, Hadith 168