莫等闲，白了少年头，空悲切。
Do not idle away your time; when your hair turns white in youth, you will grieve in vain.
—— 岳飞
Source: 满江红(Man Jiang Hong)
Background
From Yue Fei's "Man Jiang Hong," expressing the poet's feelings about the passage of time and unfulfilled ambitions.
Interpretation
Don't waste time idly, only to regret when your hair turns gray. One should strive to achieve ideals and ambitions while young and capable.