Sahih Bukhari Volume 4, Book 56, Hadith Number 774.

Narated By Anas bin Malik : The Prophet went out on one of his journeys with some of his companions. They went on walking till the time of the prayer became due. They could not find water to perform the ablution. One of them went away and brought a little amount of water in a pot. The Prophet took it and performed the ablution, and then stretched his four fingers on to the pot and said (to the people), “Get up to perform the ablution.” They started performing the ablution till all of them did it, and they were seventy or so persons.