Musnad Abu Ya'la al-Mawsili 153:199

Yahya ibn Ayyub narrated to us, Salim ibn Salim narrated to us, from Ali ibn Urwah, from Muhammad ibn al-Munkadir, from Ibn Umar, from the Prophet, may Allah bless him and grant him peace, who said: "Whoever leads a blind person for forty steps, Paradise becomes mandatory for him."