The billing on iOS is handled by Apple. Apple will send you a receipt of your subscription via e-mail. Often it can take 3-5 days to arrive. In case you have e-mail notifications turned off, you can get your receipt by following the steps:

  1. Open the App Store.
  2. Tap your profile icon.
  3. Tap your name at the top of the profile page.
  4. Scroll down and choose Purchase History.
  5. Find the order and tap the "Total Billed" label to see the transaction details.
  6. Tap Resend to have it emailed to you again.