ช่วยดู Code Warning: Cannot modify header information - headers already sent by หน่อยครับ

<?php
ob_start(); 
if(isset($_POST["Username"]) && isset($_POST["Password"])){
	$Username = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["Username"]);
	$Password = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["Password"]);
	$md5Password = md5($Password);
	$sql = mysql_query("SELECT id FROM account WHERE Username='$Username' AND Password='$md5Password' LIMIT 1");
	$userCount = mysql_num_rows($sql);
	if($userCount == 1){
		while ($row=mysql_fetch_array($sql)) {
			$id = $row["id"];
		}
		$_SESSION["id"] = $id;
		$_SESSION["Username"] = $Username;
		$_SESSION["Password"] = $Password;
		header("location: account.php");
		exit();
	}
	echo "in";
}

?>