Sahih Bukhari Volume 7, Book 62, Hadith Number 14.

Narrated By ‘Aisha  : I said, “O Allah’s Apostle! Suppose you landed in a valley where there is a tree of which something has been eaten and then you found trees of which nothing has been eaten, of which tree would you let your camel graze?” He said, “(I will let my camel graze) of the one of which nothing has been eaten before.” (The sub-narrator added: ‘Aisha meant that Allah’s Apostle had not married a virgin besides herself.)