Musnad Ahmad 30:959

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Hammam ibn Munabbih, that he heard Abu Hurayrah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The prayer of one who has hades (ritual impurity) is not accepted until he performs wudu (ablution)." A man from the people of Hadramawt said to him: "What is hades, O Abu Hurayrah?" He said: "Passing wind silently or passing wind audibly."