Musnad Ahmad 30:3621

'Abd al-Samad narrated to us, 'Abd al-Rahman—meaning Ibn 'Abd Allah ibn Dinar—narrated to us, he said: I heard my father mentioning, from Abu Salih, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him: "A man saw a dog eating the moist earth out of thirst. So the man took his leather sock and began to scoop water for it with it until he quenched its thirst. So Allah, the Mighty and Majestic, thanked him for it and admitted him into Paradise."