Musnad Ahmad 1153:2041

Mu'adh ibn Mu'adh narrated to us, 'Ikrimah ibn 'Ammar narrated to us, from 'Abdullah ibn 'Ubayd ibn 'Umayr, from 'Aishah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to scrape the semen from his garment with the sweat of the *idhkhir* (lemon grass), then he would pray in it. And he would scrape it from his garment while it was dry, then he would pray in it."