Musnad Ahmad 759:2

Waki’ narrated to us, Ibn Abi Layla narrated to us, from his brother ‘Isa ibn ‘Abd al-Rahman, from his father ‘Abd al-Rahman, from his grandfather, who said: We were with the Prophet, may Allah bless him and grant him peace, when al-Hasan ibn ‘Ali came crawling until he climbed onto his chest, and he urinated on him. He said: So we rushed to take him, but the Prophet, may Allah bless him and grant him peace, said: "My son, my son." He said: Then he called for water and poured it over it.