Sunan Ibn Majah, Volume 5, Book 33, Hadith 3788 (DarusSalam) ⓘ
It was narrated that Anas bin Malik said: "The Messenger of Allah said: Qul Huwa Allahu ahad [Say: He is Allah, (the) One] is equivalent to one third of the Quran."
حَدَّثَنَا الْحَسَنُ بْنُ عَلِيٍّ الْخَلاَّلُ، حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ، عَنْ جَرِيرِ بْنِ حَازِمٍ، عَنْ قَتَادَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " {قُلْ هُوَ اللَّهُ أَحَدٌ} تَعْدِلُ ثُلُثَ الْقُرْآنِ " .
also referenced under: Hadith 3788 (Reference) & Book 33, Hadith 133 (In-Book)