Musnad Ahmad 30:1843

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Sa'd ibn Ibrahim, who said: I heard Abu Salamah narrating from Abu Hurairah, from the Prophet, may Allah bless him and grant him peace, that he said: "Come to the prayer while you are upon tranquility. So pray what you have caught, and complete what you have missed."