Musnad Ahmad 96:2

Ibrahim ibn Abi al-Abbas narrated to us, Baqiyyah narrated to us, Buhayr ibn Sa'd narrated to us, from Khalid ibn Ma'dan, from some of the companions of the Prophet, may Allah bless him and grant him peace: That the Messenger of Allah, may Allah bless him and grant him peace, saw a man praying, and on the back of his foot was a spot the size of a dirham that the water had not reached. So the Messenger of Allah, may Allah bless him and grant him peace, commanded him to repeat the ablution.