I am looking for the source and context of this quote, found e.g. at [St Andrews](http://mathshistory.st-andrews.ac.uk/Biographies/Rudio.html):

>It is only with the greatest difficulty that one is able to follow the writings of any author preceding Euler, because it was not yet known how to ***let the formulae speak for themselves***. This art Euler was the first to teach.<br>
— F. Rudio

(My emphasis. Slight variants suggest that it could originally have been in another language.)