Musnad Ahmad 938:19

Waki’ narrated to us, Abdullah bin Sa’id bin Abi Hind narrated to us, from Salim Abu an-Nadr, from Busr bin Sa’id, from Zayd bin Thabit: That the Prophet, peace and blessings of Allah be upon him, was in a chamber, and he would come out and pray in it. His companions noticed him, so they would pray with his prayer.