Musnad Ahmad 32:967

'Abd al-Rahman ibn Mahdi narrated to us, Shu'bah narrated to us, from Anas ibn Sirin, who said: I heard Anas say: "The Prophet, peace and blessings of Allah be upon him, had a mat sprinkled [with water] for him, and he prayed upon it." He said: A man said to him: "Did you see him pray the Duha [mid-morning] prayer?" He said: "I did not see him [do so] except on that day."