يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا قُمْتُمْ إِلَى الصَّلَاةِ فَاغْسِلُوا وُجُوهَكُمْ وَأَيْدِيَكُمْ إِلَى الْمَرَافِقِ
Meaning: O believers, when you rise for prayer, wash your faces and your hands up to the elbows.
The imperative fagh-silu directs washing. The words ila al-marafiq state the terminal limit of washing the arms.