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 .
• Sunan an-Nasa'i, 916
• Sunan an-Nasa'i, Vol. 2, Book of The Commencement of the Prayer, Hadith 917
• Sunan an-Nasa'i, Book of The Commencement of the Prayer, Hadith 917