Language:
كتاب الوضوء
4
Ablutions (Wudu')
(40)
Chapter: The using of the remaining water after ablution
(40)
باب اسْتِعْمَالِ فَضْلِ وَضُوءِ النَّاسِ
وَأَمَرَ جَرِيرُ بْنُ عَبْدِ اللَّهِ أَهْلَهُ أَنْ يَتَوَضَّئُوا بِفَضْلِ سِوَاكِهِ.

Abu Musa said:

The Prophet asked for a tumbler containing water and washed both his hands and face in it and then threw a mouthful of water in the tumbler and said to both of us (Abu Musa and Bilal), "Drink from the tumbler and pour some of its water on your faces and chests."
وَقَالَ أَبُو مُوسَى دَعَا النَّبِيُّ صلى الله عليه وسلم بِقَدَحٍ فِيهِ مَاءٌ، فَغَسَلَ يَدَيْهِ وَوَجْهَهُ فِيهِ، وَمَجَّ فِيهِ ثُمَّ قَالَ لَهُمَا اشْرَبَا مِنْهُ، وَأَفْرِغَا عَلَى وُجُوهِكُمَا وَنُحُورِكُمَا‏.‏
Reference : Sahih al-Bukhari 188
In-book reference : Book 4, Hadith 54
USC-MSA web (English) reference : Vol. 1, Book 4, Hadith 187
  (deprecated numbering scheme)