Laws of Cricket 1947 Code (Fourth Edition) - Law 33
A Batsman may retire at any time, but may not resume his innings without the consent of the opposing Captain, and then only on the fall of a wicket.
Note on Law 33
1. When a batsman has retired owing to illness, injury, or some other unavoidable cause, his innings is recorded as "Retired, Not Out", but otherwise as a completed innings to be recorded as "Retired, Out".
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