[Avodah] 10 B'Teves on Shabbos

[Danny Schoemann]

In response to one of my Halocho-a-day posts kitzur.com/7zq2
(http://halocho.blogspot.com/2009/01/halocho-244-fasting-on-shabbat-is.html)
people keep on writing in informing me that if 10 B'Teves were to
[theoretically] fall on Shabbos, we would fast.

Over the years I've heard this from various people.

Where does this Halocho come from? The SA in OC 550:3 says "if these 4
fasts fall on Shabbos that are deferred to Sunday."

Thanks

- Danny