What the Scrum Guide Says

(Bold emphasis added by me)
  • "Product Backlog item estimates are calculated initially during Release Planning, and thereafter as they are created. During Product Backlog grooming they are reviewed and revised. However, they can be updated at any time."
  • (Optional) "Tip :Teams often spend 10% of each Sprint grooming the product backlog to meet the above definition of the Product Backlog. When groomed to this level of granularity, the Product Backlog items at the top of the Product Backlog (highest priority, greatest value) are decomposed so they fit within one Sprint. They have been analyzed and thought through during the grooming process. When the Sprint Planning meeting occurs, these top priority Product Backlog items are well understood and easily selected."

(Backlog grooming supplants most of what normally happens in the first part of the Sprint Planning Meeting)
  • "The Sprint Planning Meeting consists of two parts. The first part is when what will be done in the Sprint is decided upon."
  • "Here, the Product Owner presents the top priority Product Backlog to the Team. They work together to figure out what functionality is to be developed during the next Sprint. The input to this meeting is the Product Backlog, the latest increment of product, the capacity of the Team, and past performance of the Team."