What sign did God promise to give to Eli?

That he would kill his two sons, Hophni and Phinehas, on the same day.

(They were sons of Belial who didn't know the Lord.)
Now the sons of Eli were sons of Belial; they knew not the LORD. 1 Samuel 2:12

They hearkened not unto the voice of their father, because the LORD would slay them. 2:25

And this shall be a sign unto thee, that shall come upon thy two sons, on Hophni and Phinehas; in one day they shall die both of them. 2:34