Musnad Ahmad 30:3389

Yazid narrated to us, Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The angels continue to pray for one of you as long as he remains in his place of prayer where he prayed, as long as he does not stand up or break his wudu. They say: 'O Allah, forgive him; O Allah, have mercy on him.'"