Musannaf Ibn Abi Shayba 3:4291

Narrated to us Abd al-Wahhab al-Thaqafi, from Ayyub, from Abu Qilabah, regarding a man who prays Dhuhr or Asr, then catches them [being prayed] in congregation. He said: "I do not like for him to expose himself to it. But if it is established while he is in the mosque, then let him pray."