Musnad Abu Ya'la al-Mawsili 165:3

Abu al-Rabi' al-Zahrani narrated to us, Farah ibn Fadalah narrated to us, from Abdullah ibn Amir, from Muhammad ibn Abdullah ibn Amr ibn Uthman, from his mother, Fatimah bint al-Husayn, from her father, Husayn ibn Ali, from the Prophet, may Allah bless him and grant him peace, who said: "Do not persist in looking at those afflicted with leprosy. And if you speak to them, let there be between you and them the distance of a spear's length."