Musannaf Ibn Abi Shayba 27:684

Ghundar narrated to us, from Shu'bah, from Abu Ishaq, from 'Amr ibn Sharhabil, who said: When Sa'd ibn Mu'adh was struck by the arrow on the day of the Trench, his blood began to flow onto the Prophet, may Allah bless him and grant him peace. Abu Bakr came and began to say, "Oh, the breaking of my back!" The Prophet, may Allah bless him and grant him peace, said, "O Abu Bakr." Then 'Umar came and said, "Indeed, we belong to Allah and indeed to Him we will return."