It is from Volume 7 of Famous Paintings in the Past Dynasties by Zhang Yanyuan of the Tang Dynasty. Legend goes like this：
故事出自唐代张彦远的《历代名画记》卷七，大意是说:
Once a famous artist Zhang Seng You drew four dragons on the walls of An Le Temple, Jin Ling,
有一次，著名艺术家张僧繇在金陵安乐寺墙壁上画了四条龙，
but he left the pupils of the dragons unfinished.
故意没有把眼晴画上。
Someone asked him the reason of the omission.
有人问他不画眼睛的道理，
In answer, he said that if he put the pupils therein, the dragons would become alive and fly away.
他说：“我如果把眼晴画上，那龙就要飞走了。”
The questioner did not believe it and requested him to complete the work.
问他的不相信，要求他把龙画完整。
The artist nodded assent.
这位艺术家答应了。
But as soon as the two dots were drawn, all of a sudden, there were thunder and lightning,
可是当一条龙的眼晴画上后，突然雷电交加，
and the dragon became alive and instantly flew away,
那龙真的活了，立刻向天上飞去，
while the other three without pupils still remained there.
其余没有眼睛的龙，仍留在墙上。
Based on this fairy tale, the last touch added to a drawing is said to be the act of adding pupils to a dragon,
根据这个神话，后人就把绘画上最后的重要一笔，称为“画龙点睛”，
that is to say,the last touch in a masterpiece is the most important part of a drawing, or any other important business.
就是说, 最后一部分工作是工作中最精彩、最重要的一部分。
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