The Scrum Guide clearly states
Role of Development Team: "They are self-organizing. No one (not even the Scrum Master) tells the Development Team how to turn Product Backlog into Increments of potentially releasable functionality;"
Estimation of Product Backlog Items: "The Development Team is responsible for all estimates. The Product Owner may influence the Development Team by helping it understand and select trade-offs, but the people who will perform the work make the final estimate."
Sprint Planning: "The number of items selected from the Product Backlog for the Sprint is solely up to the Development Team. Only the Development Team can assess what it can accomplish over the upcoming Sprint."
The Sprint Backlog:
"The Sprint Backlog is a forecast by the Development Team about what functionality will be in the next Increment and the work needed to deliver that functionality into a “Done” Increment."
......
"Only the Development Team can change its Sprint Backlog during a Sprint."
