Musnad Abu Ya'la al-Mawsili 166:2

Shayban narrated to us, Mahdi narrated to us, Muhammad bin Abdullah bin Abi Ya'qub narrated to us, from al-Hasan bin Sa'd, the freed slave of al-Hasan bin Ali, from Abdullah bin Ja'far, who said: The Messenger of Allah (may Allah bless him and grant him peace) seated me behind him one day. He whispered a hadith to me that I would not tell to anyone among the people. He said: "And the most beloved thing for the Messenger of Allah (may Allah bless him and grant him peace) to use for cover when needing to relieve himself was a mound or a garden of palm trees." He then mentioned the like of the hadith of Abdullah bin Muhammad.