Musannaf Ibn Abi Shayba 3:6240

Waki’ narrated to us, from Shu’bah, from Sa’d ibn Ibrahim, from Hafs ibn ‘Asim, from Zayd ibn Thabit; and from Qatadah, from Sa’id ibn al-Musayyib, from Ibn ‘Umar, from Zayd ibn Thabit, who said: The middle prayer is the Dhuhr prayer.