Citing personal communication for preliminary results
I hate to disappoint you, but you would be referencing the material which is impossible to get. The reviewers do see it similary and complain, and they are right in doing so. Personally, if there is any way whatsoever to convince the other group to write up the preliminary results and put them on a web page in time, do it. See point 7 in most harmful heristic. Probably, the best you could do now is saying &quot;to be published in [SB]&quot; where the reference is a half-hearted entry along the following lines:

[SB] John Smith and Joe Blotz, The Importance of Being Published, 2017, in preparation.

Adapt the title, of course.