The Painted Veil by William Somerset Maugham

A few days later Waddington, sitting with Kitty, a long glass of whisky and soda in his hand, began to speak to her of the convent.

‘The Mother Superior is a very remarkable woman,' he said. ‘The Sisters tell me that she belongs to one of the greatest families in France, but they won't tell me which; the Mother Superior, they say, doesn't wish it to be talked of.'