Musannaf Ibn Abi Shayba 3:858

Ibn Idris narrated to us, from Hisham, from Ibn Sirin, who said: Al-Muhajir informed me, saying: I read the letter of Umar to Abu Musa, in which were the prayer times. When it reached the Fajr—or he said: the Ghadah (morning)—it said: "Perform it while it is still dark or while it is still night-darkness, and lengthen the recitation."