Musnad Ahmad 25:755

Ali ibn Ishaq narrated to us, Abdullah informed us, Ibn Lahi'ah informed us, from Abdullah ibn Hubayrah, from Hanash, from Ibn Abbas, that the Messenger of Allah (peace and blessings of Allah be upon him) would go out and pour water [to relieve himself], then he would wipe himself with dust. So I would say: "O Messenger of Allah, water is near you." He would say: "And what makes me know? Perhaps I will not reach it."