Analysis of information sources in references of the Wikipedia article "Brethren of Purity" in English language version.
At another place a hadith is related from Hazrat 'Ayisha' (1, 358) which no Shiʻi would ever do
Susanne Diwald on the other hand would consider the Rasa'il just Sufi, not Shi'i, thus implying its Sunni character
Also, at two places (III, 489 and IV, 408) there are references to al- khulafa al - rashidun, i.e., the first four Caliphs which again is a Sunni belief