Musnad Ahmad 28:618

Mu'ammar ibn Sulayman narrated to us, Hajjaj narrated to us, from 'Amr ibn Shu'ayb, from his father, from his grandfather, who said: A man came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, a man is away and cannot find water; may he have intercourse with his wife?" He said: "Yes."