Musnad Ahmad 25:1573

Al-Khuza'i narrated to us, he said: Hammad ibn Salamah informed us, from 'Ammar ibn Abi 'Ammar, from Ibn 'Abbas, and from Thabit, from Anas, that the Prophet, may Allah bless him and grant him peace, used to deliver the sermon leaning against the trunk of a date palm tree, and he mentioned its meaning.