Surah No. 5, Al Maaeda, Part of Ayat No. 6
يَا أَيُّهَا الَّذِينَ آمَنُواْ إِذَا قُمْتُمْ إِلَى الصَّلاةِ فاغْسِلُواْ وُجُوهَكُمْ وَأَيْدِيَكُمْ إِلَى الْمَرَافِقِ وَامْسَحُواْ بِرُؤُوسِكُمْ وَأَرْجُلَكُمْ إِلَى الْكَعْبَينِ
O Believers ! When you prepare for prayer (Salat) wash your faces and your hands to the elbows; Pass your wet hands over your head and wash your feet to the ankles .
Ablution (Wuzu) is required to be done when a person is going to perform salat or recite Quran or before bathing (Gusl). It remains valid upto 24 hours but it is nullified if blood or pus is drawn, if one urinates, passes gas through anus or passes stool or falls into sleep.
How to perform the Wuzu is mentioned below in one of the Hadis :
Hadis No.1.142 Sahi Bukhari :
"Narrated `Ata bin Yasar: Ibn `Abbas (ra) performed ablution and washed his face (in the following way): He took handful of water, rinsed his mouth and washed his nose with it by putting water in and then blowing it out. He then, took another handful of water and washed his face, took another handful of water and washed his right forearm. He again took another handful of water and washed his left forearm, and passed wet hands over his head and took another handful of water and poured it over his right foot up to his ankles and washed it thoroughly and similarly took another handful of water and washed thoroughly his left foot up to the ankles and said, " I saw Allahs Apostle (saws) performing ablution in this way."