Musannaf Ibn Abi Shayba 3:4068

Waki’ narrated to us, from Fudayl ibn Ghazwan, from Nafi’, from Ibn Umar, that he came to the people while they were in prayer, and he had not prayed the two rak'ahs. So he entered [the prayer] with them, then he sat in his place of prayer. When the morning had advanced, he stood up and performed them.