Musannaf Ibn Abi Shayba 18:258

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Shu’bah, from Qatadah, from ‘Ikrimah, from Ibn ‘Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "This and this are equal," meaning the little finger and the thumb.