Musannaf Ibn Abi Shayba 3:2396

Abu Usama narrated to us, from Sa'id, from Qatada, from Sa'id ibn al-Musayyib and al-Hasan, they both said regarding a man who entered into the Asr prayer with a group of people while he thought they were in the Dhuhr prayer, and then it turned out they were in the Asr prayer: He must repeat both prayers entirely.