Sahih Muslim
The Book of Menstruation (Kitab Al-Haid)
Book Number: 003 / Hadith Number: 673
'Abdullah b. Ja'far reported: The Messenger of Allah (may peace be upon him) one day made me mount behind him and he confided to me something secret which I would not disclose to anybody; and the Messenger of Allah (may peace be upon him) liked the concealment provided by a lofty place or cluster of dates (while answering the call of nature), Ibn Asma' said in his narration: It implied an enclosure of the date-trees.