How to wash the arms when doing udu
Question

I want to ask about how the arms are to be washed when doing wudu. I know, thanks to Allah and then to you, that washing the arms should be from the tips of the fingers to the elbow, but my question is: is washing it from the tips of the fingers to the elbow regarded as one time, then washing it from the elbows to the tips of the fingers is regarded as a second time, or is this procedure regarded as one time only? May Allah reward you with good.
Praise be to Allah.One of the obligatory parts of wudu without which it is not valid is washing the arms from the fingertips to the elbow once, but it is mustahabb to wash them three times. 
What is meant by once is that the water should reach all of the arm, no matter how it is done. If the water reaches all of the arm from the tips of the fingers to the elbows, then the arm has been washed once. 
If the water reaches the arm a second time from the elbow to the fingertips and so on, then this is a second time. 
See: al-Fawakih al-Dawaani, 1/144.