The book "Hitler: 1889-1936 Hubris" by Ian Kershaw  

From the chapter “The Drummer” on page 193 “In the feverish atmosphere in the Lowenbraukeller the same evening, he was given s similar hero’s welcome as he entered the hall, deliberately late, shielded by his bodyguard, arm outstretched in the salute - probably borrowed from the Italian Fascists (and by them from Imperial Rome) - which would become standard in the Movement by 1926. footnote 151" referring to 1923 ?        

From page 294 “....the fascist-style salute increasingly used since 1923 and compulsory within the movement since 1926. footnote 222"

Kershaw seems to sense a lack of support for the assertions with his use of the words "probably" and "fascist-style" and of course the complete lack of support or reference for the bald claims.