מֹ֧אזְנֵי צֶ֣דֶק אַבְנֵי־צֶ֗דֶק אֵ֥יפַת צֶ֛דֶק וְהִ֥ין צֶ֖דֶק יִהְיֶ֣ה לָכֶ֑ם אֲנִי֙ יְהוָ֣ה אֱלֹֽהֵיכֶ֔ם אֲשֶׁר־הוֹצֵ֥אתִי אֶתְכֶ֖ם מֵאֶ֥רֶץ מִצְרָֽיִם׃
Just balances, just weights, a just ephah, and a just hin, shall ye have: I am the LORD your God, who brought you out of the land of Egypt.
Rashi on Leviticus
אבני צדק are stones used for weights wherewith one weighs.
Siftei Chakhamim
That is a dry measure. Because it says [regarding manna] (Shemos 16:36), “The omer is a tenth of an eiphoh.”