Musnad Ahmad 1153:1075

Yazid narrated to us, he said: Yahya informed us, he said: I heard Abdullah ibn Amir ibn Rabi'ah narrating that Aisha used to narrate that the Messenger of Allah (peace and blessings of Allah be upon him) stayed awake one night while she was at his side. She said: I said: "What is the matter with you, O Messenger of Allah?" She said: He said: "I wish a righteous man from my companions would guard me tonight." She said: While I was in that state, I heard the sound of weapons. He said: "Who is this?" He said: "I am Sa'd ibn Malik." He said: "What brought you?" He said: "I came to guard you, O Messenger of Allah." She said: Then I heard the snoring of the Messenger of Allah (peace and blessings of Allah be upon him) in his sleep.