Musannaf Ibn Abi Shayba 3:3594

Waki’ narrated to us, from Sufyan, from Mansur, from Ibrahim, who said: They used to count as the Sunnah: four before the Dhuhr, two after it, two after the Maghrib, two after the Isha, and two before the Fajr. Ibrahim said: And they used to consider two rak'ahs before the Asr as recommended, except that they did not count them as part of the Sunnah.