Musannaf Ibn Abi Shayba 3:4279

Waki’ narrated to us, he said: ‘Umar ibn Hassan al-Muslimi narrated to us, from ‘Abd al-Rahman, he said: I, Ibrahim al-Nakha’i, and ‘Abd al-Rahman ibn al-Aswad prayed Maghrib. Then we came to the mosque while they were in the Maghrib prayer, so we entered with them and prayed. When the Imam performed the taslim, I and ‘Abd al-Rahman ibn al-Aswad sat, and Ibrahim stood up and performed one rak’ah to complete [his prayer].