Musnad Abu Ya'la al-Mawsili 6:39

Zuhayr narrated to us, Waki’ narrated to us, Hisham ibn ‘Urwah narrated to us, from his father, from Sa’d, that the Prophet (may Allah bless him and grant him peace) said to him: "The third, and the third is large," or "The third is much, in the bequest."