Sahih Muslim Book 31, Hadith Number 5926.

‘Aisha reported that Allah’s Messenger (may peace he upon him) laid down on bed during one night on his arrival at Medina and said: Were there a pious person from amongst my Companions who should keep a watch for me during the night? She (‘Aisha) reported: We were in this state that we heard the clanging noise of arms lie (the Holy Prophet) said: Who is it? He said: This is Sa’d b. Abi Waqqas. Allah’s Messenger (may peace be upon him) said to him: What brings you here? Thereupon he said: I harboured fear (lest any harm should come to) Allah’s Messenger (may peace be upon him), so I came to serve as your sentinel. Allah’s Messenger (may peace be upon him) invoked blessings upon him. He then slept. This hadith has been transmitted on the authority of Ibn Rumh with a slight variation of wording.