Musannaf Ibn Abi Shayba 3:4542

Zayd ibn Hubab narrated to us, from Harun ibn Ibrahim, who said: I asked al-Hasan, and I said: "Should I pray [voluntary prayers] upon my mount?" He said: "Pray upon it." I said: "Should I perform the Witr prayer upon my mount?" He said: "No." And Ibn Sirin said: "Perform the Witr prayer upon the ground."