What did the Rechabites refuse when Jeremiah set it before them?

Answer: Wine

"We do not drink wine," they said, because their forefather Jonadab son of Rechab had commanded them never to drink it.

“Then I set pitchers full of wine and some cups before the men of the house of the Rechabites, and I said to them, “Drink some wine.” “We do not drink wine,” they replied, “for our forefather Jonadab son of Rechab commanded us, ‘Neither you nor your descendants are ever to drink wine.”— Jeremiah 35:5-6 (BSB)