Sahih Bukhari Volume 3, Book 36, Hadith Number 465.

Narated By ‘Aisha : (The wife of the Prophet) Allah’s Apostle and Abu Bakr hired a man from the tribe of Bani-Ad-Dil as an expert guide who was a pagan (follower of the religion of the pagans of Quraish). The Prophet and Abu Bakr gave him their two riding camels and took a promise from him to bring their riding camels in the morning of the third day to the Cave of Thaur.