Musannaf Ibn Abi Shayba 1:445

Ibn ‘Ulayyah narrated to us, from Yunus, from al-Hasan, that the Prophet, may Allah bless him and grant him peace, saw a man who had left a spot the size of a fingernail on his foot [unwashed]. The Messenger of Allah, may Allah bless him and grant him peace, said to him, "Perform your ablution well." Yunus said: So al-Hasan used to wash that place.