Musannaf Ibn Abi Shayba 3:4044

Waki’ narrated to us, from Mis’ar, from Abu al-Walid ibn Abi Malik, from Abu ‘Ubayd Allah, from Abu al-Darda’, who said: I would come to the people while they were in rows for the dawn prayer, so I would pray the two rak’ahs, then I would join them.