Musannaf Ibn Abi Shayba 3:6373

Waki’ narrated to us, from Yazid ibn Ibrahim, who said: I heard al-Hasan saying: "The 'adbar al-nujum' (the retreating of the stars) are the two rak'ahs before the dawn prayer, and the 'adbar al-sujud' (the ends of the prostrations) are the two rak'ahs after the Maghrib prayer."