Sunan an-Nasa'i, Hadith 916 ⓘ
It was narrated that Ibn Abbas said: Concerning the words of Allah, the Mighty and Sublime: "Seven of Al-Mathani (seven repeatedly-recited): "The seven long ones."
أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ حَدَّثَنَا شَرِيكٌ، عَنْ أَبِي إِسْحَاقَ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسٍ، فِي قَوْلِهِ عَزَّ وَجَلَّ { سَبْعًا مِنَ الْمَثَانِي } قَالَ السَّبْعُ الطُّوَلُ .
also referenced under: Book 11, Hadith 41 (In-Book) & Volume 2, Book 11, Hadith 917 (DarusSalam)