beat a dead horse
To waste time doing something that has already been tried.
“Do you think it’s worth sending my manuscript to other publishers or I am just beating a dead horse?“
get something straight from the horse’s mouth
To get information directly from the source of information.
“I heard that Kyle will quit school but I don’t believe it, it’s not like him. I won’t believe it until I hear straight from the horses mouth.“
(put) the cart before the horse
To do something before you should have. Specifically when it is smart to wait.
A: “I heard the city has approved construction of the new baseball stadium.“
B: “Really? The pro team hasn’t even agreed to come to our city! I think their putting the cart before the horse.”