Musnad Ahmad 33:842

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Amr ibn Dinar, who said: I heard Jabir narrating that the Prophet, may Allah bless him and grant him peace, delivered a sermon and said: "If one of you comes while the Imam has already gone out [to deliver the sermon], let him pray two rak‘ahs."