What phrase from romeo and Juliet makes an allusion to greek myth?

The allusion to Greek myth in Romeo and Juliet is the phrase "the course of true love never did run smooth," found in Act I, Scene I. This line is a reference to the myth of Hero and Leander, a Greek love story about two young lovers who were separated by the Hellespont Strait. Leander would swim across the strait every night to meet Hero, but one stormy night, he drowned and Hero killed herself in grief. The phrase "the course of true love never did run smooth" suggests that love is often marked with obstacles and difficulties.