Hazrat Sheikh Ml Ibrahim Mia Rahamatullah alyah used to advise us to stand and drink the left over wudhu water. Thereafter he advised us to sprinkle some of the left over wudhu water on our lower garment. This is a Sunnat practice. Sheikh used to explain that it will help in controlling the the tongue and private parts from misuse.
In our country, South Africa, this is not a common practice and sometimes people used to object to the above.
Hazrat Justice Mufti Taqi Usmani DB, writes in his book, "The Great Scholars of the Deoband Islamic Seminary," the view of Sheikhul Hind Ml Mahmud al Hasan Rahmatullah. The excerpt appears below:
It is reported in almost all of the hadith collections that after completing wudu, the Prophet Sallalahu Alayhi Wasalam used to drink the leftover water from the container3. It has also been reported that he used to sprinkle some of the water on his tahband (lower garment)4. The ‘ulama' have offered a variety of interpretations concerning the wisdom of these two practices. However, Mufti Muhammad Shafi‘ maintained that Shaykh al-Hind’s explanation for these was the most insightful one.
According to his view, wudu is an action for ensuring the cleanliness and purity of the bodily parts. Yet, while it is important to purify the outward, it is even more important to purify the inward. So, after completing wudu, the Prophet Sallalahu Alayhi Wasalam performed these particular actions, both of which are connected to the matter of inner purity. The connection is that the two body parts involved here, the tongue and the private parts, go to the very foundations of humanity’s most negative traits and wrongdoings. Therefore, the Prophet Sallalahu Alayhi Wasalam is recorded as having said:
"He who guarantees that he will protect the following two: that which lies between his jaws (i.e. his tongue) and that which lies between his legs (i.e. his private parts), I will assure him of entry into Paradise.5"
Hence, the purpose of drinking the water left over from wudu, and sprinkling some of it on one’s lower garment, is to focus attention on the importance of protecting against these two major sources of human deficiency.
3 Tirmidhi, Nasai, Tabaran'i’s al-Mu‘jam al-Kabir and Ahmad.
4 Abu Dawud, Ahmad and al Hakim
5 Al Bukhari