Sahih Bukhari Volume 009, Book 090, Hadith Number 337.
Narrated By 'Aisha : One night the Prophet was unable to sleep and said, "Would that a righteous man from my companions guarded me tonight." Suddenly we heard the clatter of arms, whereupon the Prophet said, "Who is it?" It was said, "I am Sa'd, O Allah's Apostle! I have come to guard you." The Prophet then slept so soundly that we heard him snoring. Abu 'Abdullah said: 'Aisha said: Bilal said, "Would that I but stayed overnight in a valley with Idhkhir and Jalil (two kinds of grass) around me (i.e., in Mecca)." Then I told that to the Prophet.