Canada's constitution, being a 'mixed' or hybrid constitution (a constitution that is partly formally codified and partly uncodified) originally did not make any reference whatsoever to a prime minister, with her or his specific duties and method of appointment instead dictated by "convention". In the Constitution Act, 1982, passing reference to a "Prime Minister of Canada" is added, though only regarding the composition of conferences of federal and provincial first ministers.
Which law first referred to the prime minister in Canada?
the Constitution Act