Musnad Ahmad 962:147

Ali ibn Ishaq narrated to us, Ibn al-Mubarak informed us, Yahya ibn Ayyub narrated to us, from Ubayd Allah ibn Zahr, from Ali ibn Yazid, from al-Qasim, from Abu Umamah, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever strokes the head of an orphan boy or an orphan girl, not stroking it except for the sake of Allah, will have for every hair that his hand passes over, good deeds. And whoever treats an orphan girl or an orphan boy who is with him with kindness, I and he will be in Paradise like these two," and he joined his two fingers.