The Messenger of Allah ﷺ said, "Say, He is Allah, the One, equals a third of the Quran."
حدثنا أمية بن خالد قال حدثنا محمد بن عبد الله بن مسلم ابن أخي الزهري عن عمه الزهري
قال رسول الله ﷺ قل هو الله أحد تعدل ثلث القرآن
Its isnad is sahih. Umaya bin Khalid is Ibn Aswad Qaysi, and he is trustworthy, his hadith appearing in Muslim and others. The hadith was narrated by Nasai in Amal al-Yawm, number 695, and by Malik in Muwatta 1/209, number 19, in the chapter on Quran, concerning what has been said about "Say, He is Allah, the One" [Ikhlas 112:1], and it has already appeared at 9501.