Each of the Aseres HaDibros, generally speaking, begin and end at consecutive Parshah breaks. Nevertheless, the first and second are combined into one parshah, while the last one is split into two. This is true in both sets, in Yisro and Va'Eschanan. How do we know that Anochi HaShem and Lo Yihiyeh aren't one Dibbur, with Lo Tirtzach rounding out the first five, and the ninth and tenth each being Lo Sachmod/Sisaveh?
I'm looking for a source that demonstrates that the Dibros must start and end where we know they do. Not just one that says it, but one that weighs the different possibilities until coming to the correct conclusion.
(Granted there have to be ten Dibros - they are referred to as such in Shemos 34:28.)