Sahih Bukhari Volume 4, Book 52, Hadith Number 136.

Narated By ‘Aisha : The Prophet was vigilant one night and when he reached Medina, he said, “Would that a pious man from my companions guard me tonight!” Suddenly we heard the clatter of arms. He said, “Who is that? ” He (The new comer) replied, ” I am Sad bin Abi Waqqas and have come to guard you.” So, the Prophet slept (that night).