Mishkat al-Masabih 2 Knowledge كتاب العلم 35

وَعَنِ ابْنِ مَسْعُودٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أُنْزِلَ الْقُرْآنُ عَلَى سَبْعَةِ أَحْرُفٍ لِكُلِّ آيَةٍ مِنْهَا ظَهْرٌ وَبَطْنٌ وَلِكُلِّ حَدٍّ مَطْلَعٌ» رَوَاهُ فِي شَرْحِ السُّنَّةِ

Ibn Mas’ud reported God’s messenger as saying, “The Qur’an was sent down in seven modes. Every verse in it has an external and an internal meaning, and every interdiction has a source of reference.” Baghawi transmitted it in Sharh as-sunna .