Bernie Sanders totally sold out to Crooked Hillary Clinton. All of that work, energy and money, and nothing to show for it! Waste of time.— Donald J. Trump (@realDonaldTrump) July 26, 2016
Sanders responded in no uncertain words.
Essentially, this is an attempt to replicate the success Clinton had with her own response to Trump on Twitter earlier this year, when she told Trump, "Delete your account." As Dara Lind explained for Vox, this is a pretty standard joke tweet — not typically meant literally, but usually meant to convey a total rejection of what someone says.
Similarly, Sanders’s tweet is trying to latch on to another Twitter meme. On Twitter, it’s very popular to say "never tweet" — particularly among journalists — as a reference to people getting into serious trouble, even losing their jobs, for tweeting bad things.
Sanders was suggesting that Trump’s tweet is so ridiculous that it’s not worth taking seriously — to the point that it would be the kind of tweet that could possibly cost him a job.