Musannaf Ibn Abi Shayba 3:4421

Ghundar narrated to us, from Shu'bah, who said: I asked Hammad about a man who did not perform Witr until the sun rose. He said: "It is more beloved to me that he performs Witr." And I asked al-Hakam, and he said: "If he wishes, he may not perform Witr."