I addition to some of the textual variants cited in the other answers, two of the ancient Aramaic translations also include the missing phrase "Let's go out to the field".

**Targum Pseudo-Jonathan**

> ואמר קין לות הבל אחוהי איתא ונפוק תרוינן לברא

**Targum Yerushalmi**

> ואמר קין להבל אחוי איתא ונפוק לאפי ברא

However, this is not necessarily evidence of an original Hebrew text with this phrase, as the Aramaic translations often add in explanations that are not translations of the actual text.

As to the premise that the word ויאמר cannot be a standalone word for talking (without specifying what was said), this is not true. As [Samuel David Luzzato][1] points out, we have a clear example of this in Exodus 19:25.

> וַיֵּרֶד מֹשֶׁה אֶל הָעָם וַיֹּאמֶר אֲלֵהֶם
> 
> So Moses went down to the people and told them. ([ESV][2])

Like many other commentators, Luzzato explains that our verse means that Cain told Abel what God had just told him in the previous two verses.


Even the Septuagint, the Vulgate, the Samaritan Pentateuch, and the Syriac Bible – which add in the text that Cain spoke to Abel – do not add any text in Exodus 19:25.

**Septuagint**

> κατέβη δὲ Μωυσῆς πρὸς τὸν λαὸν καὶ εἶπεν αὐτοῖς.
> 
> And Moses went down to the people, and spoke to them. ([Brenton][3])


**Vulgate**

> *descendit Moses ad populum et omnia narravit eis*
> 
> And Moses went down to the people and told them all.
> ([Douay-Rheims][4])


**Samaritan Pentateuch** 

> וירד משה מן ההר אל העם ויאמר אליהם ([STEP][5])

> So Moses went down from the mountain unto the people, and spake unto
> them. ([STEP][6])

**Syriac Bible**

> ܘܢܚܼܬ ܡܘܫܐ ܠܘܬ ܥܡܐ ܘܐܡܼܪ ܠܗܘܢ܂ ([CAL][7])

While the Targum Pseudo-Jonathan and Targum Yerushalmi do in fact add in words in Exodus 19:25, as I mentioned above this does not prove that there was a variant Hebrew text since the targums often insert explanatory words beyond the exact translation.


  [1]: https://en.wikipedia.org/wiki/Samuel_David_Luzzatto
  [2]: https://www.biblestudytools.com/esv/exodus/19.html
  [3]: http://ecmarsh.com/lxx/Exodus/index.htm
  [4]: http://vulgate.org/ot/exodus_19.htm
  [5]: https://www.stepbible.org/?q=version=SP%7Creference=Exod.19&options=VNHUG
  [6]: https://www.stepbible.org/?q=version=SPE%7Creference=Exo.19
  [7]: http://cal.huc.edu/get_a_chapter.php?file=62002&sub=19&cset=U