Musannaf Ibn Abi Shayba 3:6376

Abu al-Ahwas narrated to us, from Abu Ishaq, from al-Harith, from Ali, who said: The "dubar al-sujud" (the ends of the prostrations) are two rak'ahs after the Maghrib prayer. And the "adbar al-nujum" (the setting of the stars) are two rak'ahs before the Fajr prayer.