Musnad Ahmad 25:539

Hassan ibn Musa narrated to us, Zuhayr Abu Khaythama narrated to us, from Abdullah ibn Uthman ibn Khuthaym, from Sa’id ibn Jubayr, from Ibn Abbas: That the Messenger of Allah (peace and blessings of Allah be upon him) placed his hand upon my shoulder—or upon my shoulder blades, Sa’id was in doubt—then he said: "O Allah, grant him understanding in the religion and teach him the interpretation."