Musnad Ahmad 1101:12

Uthman ibn Umar narrated to us, Malik informed us, from Salim Abu al-Nadr, from Sulayman ibn Yasar, from al-Miqdad ibn al-Aswad, that he asked the Messenger of Allah (peace and blessings of Allah be upon him) about a man who draws near to his wife and emits *madhy* (pre-seminal fluid). He said: "If one of you finds that, let him sprinkle water on his private part"—he said: meaning he should wash it—"and let him perform ablution as he performs it for prayer."