By William Shakespeare
Let me not to the marriage of true minds Admit impediments. Love is not love Which alters when it alteration finds, Or bends with the remover to remove. O no! it is an ever-fixed mark That looks on tempests and is never shaken; It is the star to every wand'ring bark, Whose worth's unknown, although his height be taken. Love's not Time's fool, though rosy lips and cheeks Within his bending sickle's compass come; Love alters not with his brief hours and weeks, But bears it out even to the edge of doom. If this be error and upon me prov'd, I never writ, nor no man ever lov'd.
This sonnet was first published in 1609. In the 1609 Quarto it is mis-numbered as 119. It is now in the public domain. Wikipedia has a reasonable description of its meaning as well as a short description of why scholars believe it is not, actually, a love poem.