Musannaf Ibn Abi Shayba 35:299

'Amr ibn Muhammad narrated to us, from Ibn Abi Rawwad, from Musa ibn 'Uqbah, who said: I accompanied Salim, and I lagged behind him on the road. He said, "What held you back?" I said, "I performed the Witr prayer." He said, "Why not [perform it] upon your riding beast?" And he mentioned that Abu Hanifah said: It does not suffice him to perform it upon it.