Sunan Ibn Majah 666
سنن ابن ماجه#666SahihIt was narrated that 'Umar bin Khattab said:"The Messenger of Allah saw a man performing ablution and he missed a spot the size of a fingernail on his foot. He commanded him to repeat the ablution and his prayer, so he did
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 Muslim
In-book reference: 1:400