Did he or didn't he?

We have a saying round here (and maybe in other places as well) "Put up or shut up". President Trump stated that on Tuesday last, President Assad of Syria launched a "chemical weapons attack" against "men, women and children" in the Syrian town of Khan Sheikhoun.  And this was put forward as justification for an American strike against the airfield where the planes involved were said to have taken off from.  So because President Trump said it, it must be true?  He offered no supporting evidence - but then, he has a track record of making unsupported statements.  Maybe he's right - maybe he isn't, but I am a little concerned at the way our politicians (for the most part) are queueing up to support him.  If there is evidence, then let's hear it, otherwise a healthy scepticism would seem to be in order.