Musannaf Ibn Abi Shayba 1:1290

Waki’ narrated to us, from Ibn Abi Layla, from his brother ‘Isa, from his father ‘Abd al-Rahman ibn Abi Layla, from his grandfather Abu Layla, who said: We were sitting with the Prophet (peace be upon him) when al-Husayn ibn ‘Ali came crawling until he sat upon his chest, and he urinated on him. He said: We rushed to take him, but the Prophet (peace be upon him) said: "My son, my son." Then he called for water and poured it over him.