"I hear that you were at the party last night".
or
"I heard you was at the party last night".

You being singular. Which, grammatically, is correct?

Family feud in the balance here!!!!

You has two versions: the second person singular version and the second person plural version. BOTH conjugate the same. You were (e.g. talking to your sister) / You were (e.g. talking to your friends).

You was is always wrong.