Sahih Bukhari Volume 3, Book 46, Hadith Number 724.

Narated By Abu Huraira : Allah’s Apostle said, “A pious slave gets a double reward.” Abu Huraira added: By Him in Whose Hands my soul is but for Jihad (i.e. holy battles), Hajj, and my duty to serve my mother, I would have loved to die as a slave.