Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

Allah's Messenger (SAW) as saying, "None of you ceases to be in prayer as long he waits for it. And the angels cease not to pray for him as long as he is in the mosque (0 Allah, forgive him, 0 Allah, have mercy on him), and this till he has hadath and breaks his ablution)." Then a man of Hadramawt asked, "0 Abu Hurayrah, what is hadath?" He said, "It is to break wind whether there is an accompanying sound or not." [Ahmed 8084, Ibn e Majah 799, Muslim 649]