Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
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 ".Narrated Abu Huraira:Allah's Messenger (ﷺ) said, "The Um (substance) of the Qur'an is the seven oft-repeated verses (Al-Mathani) and is the Great Qur'an (i.e. Surat-al-Fatiha)
From Sahih al-Bukhari, whose reports the scholars accept as authentic.