Musnad Ahmad 25:139

Isma'il narrated to us, Hisham informed us, he said: Yahya ibn Abi Kathir wrote to me, narrating from 'Ikrimah, that 'Umar used to say regarding [the declaration of something as] haram: "It is an oath that one must offer expiation for." Hisham said: And Yahya wrote to me, narrating from Ya'la ibn Hakim, from Sa'id ibn Jubayr, that Ibn 'Abbas used to say regarding [the declaration of something as] haram: "It is an oath that one must offer expiation for." Then Ibn 'Abbas said: "There has certainly been for you in the Messenger of Allah an excellent pattern [of conduct]" [Al-Ahzab: 21].