Musannaf Ibn Abi Shayba 3:6225

Abu Dawud al-Tayalisi narrated to us, from Ibn Abi Dhi'b, from al-Zibriqan, from Zuhrah, who said: We were sitting in the mosque with Zayd ibn Thabit, and he was asked about the middle prayer. He said: "It is Dhuhr." Then Usamah passed by and was asked, and he said: "It is Dhuhr. The Messenger of Allah (may Allah bless him and grant him peace) used to pray it during the midday heat."