What section of the constitution says the president can veto a law?

Article I, Sec. 7, 2nd paragraph. This does not specifically mention a "Veto Power" but describes the process, ie. the President can "return" a bill he/she does not approve of, with "Objections" noted. The House and Senate can reconsider it and pass the bill by a 2/3's vote and it will become law without the President's needing to sign it.