on the omud beys it says in D.H.EIN OYMDIN that a Shura cannot be less than 10 without giving the Mekor which is actually from MAS Megillah 23 OMUD Beys but on the next D.H. here in Berochos of BIRCHAS AVELIM RASHI goes out of his way to tell you that subject is discussed in Mas. KESUVOS so my HEOROH or SHAYLAH is why does Rashi give you the Mekor for BIRCHAS AVEILIM but not for EIN SHURA POCHUS MI-YUD
boruch kahan, london u.k.
Although I would not claim to have an answer for every Rashi in Shas when he brings a Mekor and when he does not, however in your case Rashi is not bringing a Mekor to the Halachah from Kesuvos. Rather, he is saying that the Nusach of the Berachah can be found in Kesuvos. This is clear from his Lashon "v'Hi Meforeshes" and not "k'd'Isa."