It is commonly stated in books (including Merriam-Webster's dictionary) that the Augean Stables hadn't been cleaned for 30 years prior to the labor of Heracles, yet none of those books list a source for that detail.
Searching on Topostexts I have been unable to find any:
Diodorus Siculus only says
These stables contained an enormous mass of dung which had accumulated over a great period
A vague "great period", not specifically thirty years.
Pseudo-Apollodorus Bibliotheca makes no mention of how long the stables went uncleaned.
Dio Chrysostom writes
But before that, to avoid creating the opinion that he did only impressive and mighty deeds, he went and removed and cleaned away the dung in the Augean stables, that immense accumulation of many years.
Not specifically thirty years.
Lucian of Samosata writes
I may not cleanse that Augean stable completely, but I will do my best, and fetch you out a few loads as samples of the unspeakable filth that three thousand oxen could produce in many years
Again, only a vague amount of years, no specific mention of thirty years.
Robert Graves, perhaps knowing there was no ancient authority for the figure of thirty years, only said
Now, the dung in Augeias's cattle yard and sheepfolds had not been cleared away for many years
Is there any source that specifically states thirty years, and if not, how did this exact number become so prevalent in books today?