The Wikipedia article about Minkowski's question mark function says that "the derivative vanishes on the rational numbers", i.e. that $?(x)=0$ for all $x\in\mathbb Q$. However, I have not been able to find an academic paper or book which contains this result. (I have only found it stated that the derivative is zero almost everywhere, but not that the "almost everywhere" includes all the rationals.) Since I need to refer to this fact in an academic paper of my own, Wikipedia won't do. Can anyone help me with a source?
See Extensive bibliography on the Minkowski Question Mark Function and allied topics compiled by Giedrius Alkauskas, which currently has 69 items. See also my math StackExchange list Bibliography for Singular Functions, which contains links to some of the papers that Alkauskas did not provide a link to, such as the papers by Denjoy.
(ADDED NEXT DAY) Regarding your request for a specific reference that the Minkowski $?(x)$ function has a zero derivative at each rational number, the (apparently unpublished) manuscript On the Minkowski measure by Linas Vepštas (arXiv:0810.1265, October 2008) states the following in its Abstract: One can show by classical techniques that its derivative must vanish on all rationals. As for a specific reference to a published proof, I suggest writing to some of the authors who have published in this area, such as Alkauskas, Bibiloni, Dushistova, Moshchevitin, Paradís, Viader, etc.