This allusion comes from Nature's Blessing in Lie Zi , the Taoist classical work collected at the beginning of the Han Dynasty：
故事出自《列子·天瑞》，大意是说:
There was a man of the State of Qi who was worrying that the sky might one day fall down,
有个杞国人老是担心天会掉下来，
the earth might sink,
地会陷下去，
and he would not know where to hide himself.
而他则不知藏身何处。
This so much troubled him that he could not eat or sleep.
于是，整天惶惶不安，以至于睡不着觉，吃不下饭。
Someone went to reassure him about this, saying,
有人就跑去开导杞国人，说:
“The sky is only formed of bright things in the air.
“天，不过就是空气罢了。
There is no place where there is no air.
天地之间到处充满了空气。
Whenever you move or breathe, you are living right in this sky.
你伸展、呼吸，整天生活在空气之中，
Why do you need ever to worry that the sky will fall down?”
为什么担心天会掉下来呢?”
The man rejoined，
杞国人明白了，可又担心地说:
“ If the sky were really nothing but air，would not the sun, the moon and the stars fall down?”
“如果天果真只是空气的话，日月星辰不就会掉下来了吗？”
And the persuader said,
那人告诉他说:
“But the sun, the moon and the stars are nothing but bright things in the air.
“日月星辰，也不过是空气中有光亮的东西，
Even if they should fall down, they would not hurt anybody.”
就算掉下来，也不能砸坏人。”
“ But what if the earth should sink?”
杞国人又说:“那地如果陷下去, 又怎么办呢?”
And the other replied, “The earth is also only formed of accumulated solids, which fill all space.
那人说:“地不过是堆积成团的东西，充满了四方空虚的地方，
There is no place where there are no solids.
大地之上到处都是这种团块。
As you walk and stamp on the ground, you are moving the whole day on this earth.
你走来走去，整天在地上行动，
Why do you ever need to worry that it may be destroyed?"
为什么担心地会陷下去呢?”
Then the man was gladly relieved,
听了这话，杞人放心地笑了起来，
so was the successful persuader.
那人也放心地笑了。
Now this allusion is used to refer to entirely unnecessary worries.
后人就用“杞人忧天”一词来比喻毫无必要的担忧。
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