Musnad Ahmad 626:38

Muhammad ibn Ubayd narrated to us, Muhammad ibn Amr narrated to us, from Abu Salamah, from al-Mughirah ibn Shu’bah, who said: I was with the Messenger of Allah, may Allah bless him and grant him peace, on one of his journeys. When he went [to relieve himself], he would go far away. He went for his need and said: "O Mughirah, follow me with water." Then he mentioned the hadith.