Musannaf Ibn Abi Shayba 3:4261

Mu'tamir narrated to us, from Ibn 'Awn, who said: I asked Nafi' about voluntary prayer during the day. He said: "As for me, I pray four [rak'ahs]." I mentioned that to Muhammad [Ibn Sirin], and he said: "Do you not pray two [rak'ahs]? I remember [it as such]."