Cain said "Let us go out into the field", as attested by LXX:
[Septuagint Genesis] 4:8
And Cain said to Abel his brother, Let us go out into the plain; and it came to pass that when they were in the plain Cain rose up against Abel his brother, and slew him.
I assume that "plain" is "sadeh", by the repetition of "sadeh" later. I prefer to translate "sadeh" as "field" rather than "plain". The Samaritan Pentateuch has in 4:8
"We'll go to the field"
And considering the identity of the remainder of the chapter to the Masoretic version, this is certainly the elided text.
I should point out that one can see just by grammar that there is omitted text--- "amar" cannot appear, just by Hebrew grammar, without spoken dialog next to it, just as "I said" is ungrammatical in English.
Highly likely that Masoretic is corrupted
It is very likely that Masoretic is corrupted, because grammar errors in J are rare, and the ones that do occur are arguably intentional. But the probability is 1 part in 20 not 1 part in 10,000, so one cannot claim certainty. The problem is that it is possible that the original did not have the text, and it was supplied later because of the grammatical awkwardness. The only argument against this is that the LXX and Semaritan agree.
I can't see a motivation for deleting the text intentionally--- especially not leaving a gaping hole like that, but it is not possible to be certain in this case, only to put a greater likelihood.
Unfortunately, certain religious traditions give a bias to the hypothesis that Masoretic is never corrupted, which means that one must be careful to counterbalance this with a certain amount of resistence, in insisting that it is still likely that it is in fact the Masoretic version which is corrupted.