Musannaf Ibn Abi Shayba 33:433

Waki’ narrated to us, from Sufyan, from ‘Amr ibn Murrah, from Salim ibn Abi al-Ja’d, from Zayd ibn Suhan, from Salman, that he performed two rak’ahs before the dawn (Fajr). He said: So I said to him (something), and he said: "Guard yourself while awake, and He will guard you while asleep."