Musnad Ahmad 1153:247

Yahya and Waki’ narrated to us, Hisham narrated to us, he said: My father informed me, from ‘Aisha, "That the Prophet, peace and blessings of Allah be upon him, was brought a young boy to perform *tahnik* (rubbing a date on the palate) for him, so he sat him in his lap, and he urinated on him. He then called for water and made it follow it." Waki’ said: "He made it follow it, and he did not wash it."