Musnad Ahmad 30:1504

Harun ibn Ma'ruf narrated to us, he said: Abdullah ibn Wahb narrated to us, from Sa'id ibn Abi Ayyub, from Nafi' ibn Sulayman, from Abd al-Rahman ibn Mihran, from Abu Hurayrah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "The one who waits for the prayer after the prayer is like the horseman whose horse has become exhausted with him in the way of Allah, upon its flank. The angels of Allah pray for him, as long as he does not commit a hadath (break his ablution) or stand up. And he is in the greatest ribat (station of guarding the frontier)."