Musannaf Ibn Abi Shayba 3:1550

Waki' narrated to us, he said: Shu'bah narrated to us, from al-Hakam and Hammad, they both said: There is no harm in a man praying one rak'ah standing and one rak'ah sitting. Then Waki' said at the end, from Shu'bah, from al-Hakam, and he did not mention Hammad.