I am sorry to say I don't recall a specific source. I do recall it being commented on in several places that I read, without much fuss or objection being made about the allegation. It was surprising news, but it surely would have explained the timing of Putin's move. I think that must have been why such a large Ukrainian armed force had been gathered near the Donbass.

Putin jumped first, quickly got to Kiev, and pinned those forces in place so they couldn't reinforce anyone, and had the rest of the invasion proceed along the east and southern borders to consolidate the Lugansk and Donetsk, hold key cities, and attain some of the main objectives. Now they are in Phase II, the encirclement of the UKR forces in the Donbass and wearing them down in a "cauldron" of barrage.