UEFA have uploaded a prominent article on their website after the treble talk for Manchester City has picked up and Celtic are front and centre as UEFA explore the potential feat.

Manchester City are likely to lift the Premier League, with Arsenal falling off the pace in recent months, and are still in the Champions League, drawing with Real Madrid after the first leg, and have a FA Cup final to play on June 3rd.
UEFA explored how many other teams have won a treble, with Celtic leading the way. The Lison Lions were the first to win a treble with a European Cup, with extra credit given to Celtic for winning the Scottish League Cup in the 66/67 season. The featured image of the article
Furthermore, UEFA goes on to say that the Lisbon Lions are the only team in history to win a quadruple that consists of a European Cup, domestic league, domestic cup and domestic league cup.
The record lasts on for this season at least, but it looks unlikely that it will ever be achieved again, with winning four major trophies in the same season near impossible.