The closing line of the Han-dynasty yuefu poem 《
长歌行》 (Long Song Ballad):
百川东到海,
何时复西归?
少壮不努力,
老大徒伤悲。 — 'All rivers flow east to the sea; when will they ever return west? If in youth and vigor you do not strive, in old age you will only grieve in vain.' Almost always cited together with the preceding line
少壮不努力.