Musannaf Ibn Abi Shayba 35:80

Waki’ narrated to us, from Ibn Abi Layla, from his grandfather Abu Layla, who said: We were sitting with the Prophet, may Allah bless him and grant him peace, when Al-Husayn ibn Ali came crawling until he sat on his chest and urinated on him. He said: We rushed to take him, but the Prophet, may Allah bless him and grant him peace, said: "My son, my son." Then he called for water and poured it over it. And he mentioned that Abu Hanifa said: "It is to be washed."