The Messenger of Allah ﷺ said, "Say, He is Allah, the One, equals a third of the Quran."
حدثنا وكيع عن سفيان عن أبي قيس عن عمرو بن ميمون
قال رسول الله ﷺ قل هو الله أحد تعدل ثلث القرآن
Its isnad is sahih. Abu Qays is Awdi, Abdul-Rahman bin Tharwan, who is considered trustworthy, and his hadith appears in Bukhari. The hadith has already been mentioned at 11331.