Musannaf Ibn Abi Shayba 3:6378

'Affan narrated to us, he said: Hammad bin Salamah narrated to us, from 'Ali bin Zayd, from Aws bin Khalid, from Abu Hurayrah, who said: "The 'idbar al-nujum' (the setting of the stars) is two rak'ahs before the dawn. And the 'idbar al-sujud' (the end of the prostrations) is two rak'ahs after the Maghrib."