Allah said;

﴿فاغْسِلُواْ وُجُوهَكُمْ﴾

(then wash your faces...) The obligation for the intention before Wudu' is proven by this Ayah;

﴿إِذَا قُمْتُمْ إِلَى الصَّلوةِ فاغْسِلُواْ وُجُوهَكُمْ﴾

(When you stand (intend) to offer the Salah then wash your faces...) This is because it is just like the Arabs saying; "When you see the leader, then stand.'' Meaning stand for him. And the Two Sahihs recorded the Hadith,

«الْأَعْمَالُ بِالنِّــيَّاتِ وَإِنَّمَا لِكُلِّ امْرِىءٍ مَانَوَى»

(Actions are judged by their intentions, and each person will earn what he intended.) It is also recommended before washing the face that one mentions Allah's Name for the Wudu'. A Hadith that was narrated by several Companions states that the Prophet said, n

«لَا وُضْوءَ لِمَنْ لَمْ يَذْكُرِ اسْمَ اللهِ عَلَيْه»

(There is no Wudu' for he who does not mention Allah's Name over it.) It is also recommended that one washes his hands before he puts his hands in the vessel of water, especially after one wakes up from sleep, for the Two Sahihs recorded that Abu Hurayrah said that the Messenger of Allah said,

«إِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ نَوْمِهِ فَلَا يُدْخِلْ يَدَهُ فِي الْإِنَاءِ قَبْلَ أَنْ يَغْسِلَهَا ثَلَاثًا، فَإِنَّ أَحَدَكُمْ لَا يَدْرِي أَيْنَ بَاتَتْ يَدُه»

(If one of you wakes up from his sleep, let him not put his hand in the pot until he washes it thrice, for one of you does not know where his hand spent the night.) The face according to the scholars of Fiqh starts where the hair line on the head starts, regardless of one's lack or abundance of hair, until the end of the cheeks and chin, and from ear to ear.