Born within the sound of Bow Bells
Bow Bells are the bells of the church of St. Mary-le-Bow, Cheapside, London. To be 'born within the sound of Bow Bells' is the traditional definition of a Cockney. These days anyone with a London accent is likely to be called a Cockney. To some ears this extends to anyone who comes from the South East of England.
The church occupies a central position in the City of London and the area that the bells can be heard in has become synonymous with 'within the city boundary'.
Richard Wittington became Mayor of London in 1392. The legend has it that he was called back to London by Bow Bells when about to leave to seek his fortune. The bells are certainly ancient - there are written records of them being rung each evening at 9pm which date back to 1469. The first citation that links Bow Bells and Cockneys is Samuel Rowland's The letting of humours blood in the head-vaine, 1600:
"I scorne ... To let a Bowe-bell Cockney put me downe."
See also, 'Cockney Rhyming Slang'.