Musnad Ahmad 1017:14

Abu Nu'aym narrated to us, Bashir narrated to us, Abdullah ibn Buraydah narrated to me, from his father, who said: The Prophet, may Allah bless him and grant him peace, came out to us one day and called out three times, then said: "O people, do you know what my likeness and your likeness is?" They said: "Allah and His Messenger know best." He said: "My likeness and your likeness is only like a people who feared an enemy coming to them, so they sent a man to scout for them. While they were in that state, he saw the enemy, so he came forward to warn them. He feared that the enemy would overtake him before he could warn his people, so he signaled with his garment: 'O people, you have been attacked! O people, you have been attacked!'" three times.