An ad for The Economist stated that he read the Economist in prison as a clever way of getting around a rule forbidding him to read the news, until authorities found him out and cancelled the subscription.
This 2001 news24 article mentions the ad:
London - A new TV advertisement for the Economist magazine started on Wednesday, based on the fact that Nelson Mandela read the weekly during his many years in prison.
The advertisement's hook is that the former South African president's subscription was cancelled by the prison authorities once they had figured out the Economist covered more than just economics - a recurring misconception the publishers would like to correct.
The well-regarded and often irreverent 158-year-old magazine, which insists on calling itself a newspaper, covers the global political economy and more and is getting a face-lift with the campaign featuring Mandela to back it up.
You can find the ad on DailyMotion here.