Sahih al-Bukhari 4704
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · The Statement of Allah "And indeed, We have bestowed upon you seven Al-Mathãni (i.e., seven repeatedly recited Verses i.e., Sürat AI-Fatiha) and the Grand Qur'an." (V.15:87)
Narrated Abu Huraira:
حَدَّثَنَا آدَمُ ، حَدَّثَنَا ابْنُ أَبِي ذِئْبٍ ، حَدَّثَنَا سَعِيدٌ الْمَقْبُرِيُّ ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " أُمُّ الْقُرْآنِ هِيَ السَّبْعُ الْمَثَانِي وَالْقُرْآنُ الْعَظِيمُ " .
Allah's Messenger (ﷺ) said, "The Um (substance) of the Qur'an is the seven oft-repeated verses (Al- Mathaini) and is the Great Qur'an (i.e. Surat-al-Fatiha).