Musannaf Ibn Abi Shayba 3:4042

Waki’ narrated to us, from Dalham ibn Salih, from Wabara, who said: I saw Ibn Umar doing it. And someone who saw him doing it twice narrated to me: He came once while they were in prayer, so he prayed them [the two rak'ahs] in a corner of the mosque. Then he came another time and prayed with them, and he did not pray them.