Musannaf Ibn Abi Shayba 3:4283

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Khasif ibn Zayd al-Taymi (al-Tamimi) narrated to us, he said: Al-Hasan narrated to us that a man entered the mosque while the Prophet (may Allah bless him and grant him peace) had already prayed. He said: "Is there no man who will stand up for this one and pray with him?" So Abu Bakr stood up and prayed with him, even though he had already prayed that prayer.