Sunan Ibn Majah 3788
سنن ابن ماجه#3788SahihIt was narrated that Anas bin Malik said:"The Messenger of Allah(ﷺ) said: Qul Huwa Allahu ahad [Say: He is Allah, (the) One] is equivalent to one third of the Quran
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Fouad Abd al-BaqiSahih
Zubair Ali ZaiSahih
In-book reference: 33:132