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Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
darimi:3471 – Abū al-Mughīrah > Ṣafwān > Iyās al-Bikālī > Nawf al-Bikālī

[Machine] "Indeed, Allah has divided the Quran into three parts, and He made 'Say, He is Allah, the One' as one-third of the Quran."  

الدارمي:٣٤٧١ – حَدَّثَنَا أَبُو الْمُغِيرَةِ حَدَّثَنَا صَفْوَانُ حَدَّثَنَا إِيَاسٌ الْبِكَالِيُّ عَنْ نَوْفٍ الْبِكَالِيِّ قَالَ

«إِنَّ اللَّهَ جَزَّأَ الْقُرْآنَ عَلَى ثَلَاثَةِ أَجْزَاءٍ فَجَعَلَ قُلْ هُوَ اللَّهُ أَحَدٌ ثُلُثَ الْقُرْآنِ»  

See similar narrations below:

Collected by Muslim, Suyūṭī
muslim:811b – [Chain 1] Isḥāq b. Ibrāhīm > Muḥammad b. Bakr > Saʿīd b. Abū ʿArūbah [Chain 2] Abū Bakr b. Abū Shaybah > ʿAffān > Abān al-ʿAṭṭār Jamīʿ

He (the Messenger of Allah) said: Allah divided the Qur'an into three parts, and he made:" Say: He, Allah is One." one part out of the (three) parts of the Qur'an.  

مسلم:٨١١b – وَحَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ أَخْبَرَنَا مُحَمَّدُ بْنُ بَكْرٍ حَدَّثَنَا سَعِيدُ بْنُ أَبِي عَرُوبَةَ ح وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا عَفَّانُ حَدَّثَنَا أَبَانٌ الْعَطَّارُ جَمِيعًا

عَنْ قَتَادَةَ بِهَذَا الإِسْنَادِ وَفِي حَدِيثِهِمَا مِنْ قَوْلِ النَّبِيِّ ﷺ قَالَ إِنَّ اللَّهَ جَزَّأَ الْقُرْآنَ ثَلاَثَةَ أَجْزَاءٍ فَجَعَلَ قُلْ هُوَ اللَّهُ أَحَدٌ جُزْءًا مِنْ أَجْزَاءِ الْقُرْآنِ