Musnad Ahmad 25:971

Bahz narrated to us, Hammam narrated to us, Qatadah informed us, from Ikrimah, from Ibn Abbas, that he used to dislike [eating] the *busr* (unripe dates) alone. He would say: "The Messenger of Allah, peace and blessings of Allah be upon him, forbade the delegation of Abdul Qays from the *muzaffat* (vessels coated with pitch), and I fear that the *busr* might be [included in that prohibition]."