Sahih al-Bukhari 167

It is related that Anas ibn Malik said, "I saw the Messenger of Allah, may Allah bless him and grant him peace, when it was near the time of 'Asr and people were looking for water [for wudu'] but could not find any. Then water was brought to the Messenger of Allah, may Allah bless him and grant him peace, and the Messenger of Allah, may Allah bless him and grant him peace, placed his hand in that vessel and told people to do wudu' from it. I saw the water gushing up from under his fingers until every last one of them had done wudu'." XXXII: On water with which human hair has been washed. 'Ata' saw no harm in using hair for thread and ropes. Also about things dogs have licked and their passing through the mosque. Az-Zuhri said, "You can do wudu' with water which has been licked by a dog when you have no other water." Sufyan [ath-Thawri] said, "This ruling derives from deep understanding of where Allah says, 'If you do not find water, then do tayammum.'" (5:6) And this is water. However, there is something doubtful about it. So one should use it to do wudu' and then do tayammum also.