Shulchan Aruch Yoreh Deah 173:19 - - - שולחן ערוך יודה דעה קעג סעיף יט
לָתֵת כ' לִטְרִין לְמִי שֶׁיַּבְטִיחַ ק ' שֶׁיֵּשׁ לוֹ בַּסְפִינָה, מֻתָּר. - To give someone 20 litrin (a type of currency) to someonne who will insure 100 which he has on a ship is permitted.
The Turei-Zahav (TAZ) there explains this to mean that the owner of the merchandise gives 20 litrin to an insurer to insure the 100 litrin worth of merchandise which is on the ship.
He goes on to explain that although the owner only pays 20L up front, and in case of loss of the merchandise the insurer will pay him back 100L, this is not considered interest, since it is considered that the insurer is selling the owner of the merchandise his acceptance of responsibility for the merchandise. Since the acceptance of responsibility takes place immediately at the time of the insurer's receipt of the money, no loan has taken place, and thus any subsequent payment of the insurance money paid by the insurer to the merchandise owner is not a repayment of debt, and therefore no ribbis problem is permitted.