Musnad Abu Ya'la al-Mawsili 111:104

Abu Ya'la informed us, Zuhayr narrated to us, Bishr ibn al-Sari narrated to us, Shu'bah narrated to us, from Qatadah, from 'Ikrimah, from Ibn 'Abbas, from the Prophet (may Allah bless him and grant him peace), who said: "The little finger and the thumb are equal."