Sayyiduna Abdullah bin Ja'far (may Allah be pleased with them both) said: One day I rode behind the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) told me something in confidence which I will not narrate to anyone. And what the Messenger of Allah (peace and blessings be upon him) most loved to screen himself with when relieving himself was a mound (a rock or the like) or a grove of date palms.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 446