OK, found the reference in Truesdell's wonderful book "The Rifle and its Development for Big Game Hunting", which by the way lists all published users of the Mannlicher Schoenauer rifles in the heyday of outdoor sports:
St. George Littledale in a letter to Denis Lyell famously wrote about a .256 Mannlicher rifle given to him by Sir Edmund Loder while preparing for a trip to Tibet. Protesting that "I have a room full of rifles and do not want any more, all Loder said was "try the Mannlicher", and like Lily Langtry and the soap, I have used no other since."

The actual quote from Lillie Langtree in the Pear's Soap advert at the time seems to be "I prefer it to any other".