Volume 7 · Book 66 · Hadith 377
Narrated 'Aisha: A boy was brought to the Prophet to do Tahnik for him, but the boy urinated on him, whereupon the Prophet had water poured on the place of urine.
— Sahih Bukhari, Volume 7, Book 66: Sacrifice on Occasion of Birth (`Aqiqa), Hadith #377