Wikipedia states, that the spin degree of freedom was first formulated by Pauli in 1924:
In 1924 Wolfgang Pauli introduced what he called a "two-valued quantum degree of freedom" associated with the electron in the outermost shell.
However, there is no reference to this quote. Does someone know the publication where Pauli introduced this quantum number for the first time? The only paper I know this one, where he formulates his exclusion principle. But this is from 1925, so one year later.