The line from "Solitary Reaper" that is most thematically to the theme of eternal art in "Ode on a Grecian Urn" is:
"Long after it was heard no more."
This line shows that the impact of the Maiden's song lingered in the speaker even after the singing had ended. The effect it had did not disappear after the lyrics ended, but rather made such an impact on the speaker that he felt like he carried them around in his heart.
This relates to the theme presented in "Ode on a Grecian Urn" that art is timeless. In this ode, Keats wants to show how people will always find beauty and meaning in art, and that, once it has been appreciated, the feelings it produced will stay with us forever.