Musnad Ahmad 1018:63

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Asim ibn Kulayb, from ‘Iyad ibn Marthad, or Marthad ibn ‘Iyad, from a man among them, that he asked the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, inform me of a deed that will cause me to enter Paradise." He said to him several times: "Is either of your parents alive?" He said: "No." He said: "Then provide water." He said: "How do I provide it?" He said: "Take care of their equipment when they are present, and carry it to them when they are absent from it."