Reference for Shevuot 50:4
אליבא דרבי יהודה בן בתירא כולי עלמא לא פליגי השתא לאו והן לא בעי להבא ולשעבר בעי
Raba raised an objection [We learnt:] What is a vain oath? If he swore that which is contrary to the facts known to man, saying of a pillar of stone that it was of gold. And Ulla said: Provided that it was already known to three men [that it was of stone]. Now, the reason [that he is liable for a vain oath] is because it is known [to three men that it is of stone], but if it were not known [to three men], he would be transgressing an oath of utterance. Why? It is not [applicable in the future: T swear] it will be of gold!' He himself put the question - and he himself answered it: If it is known, he transgresses a vain oath; if it is not known, he transgresses a false oath. <br>