Musnad Ahmad 25:1020

Yahya ibn Adam narrated to us, Zuhayr narrated to us, from Abdullah ibn Uthman ibn Khuthaym, who said: Sa’id ibn Jubayr informed me that he heard Ibn Abbas saying: The Messenger of Allah (peace and blessings of Allah be upon him) placed his hand between my shoulder blades—or he said: upon my shoulders—and said: "O Allah, grant him understanding in the religion and teach him the interpretation."