From Snopes
Did Trump Say ‘If You’re Innocent, Why Are You Taking the 5th Amendment?’
Trump made this remark during a September campaign rally in Council Bluffs, Iowa during the 2016 presidential election campaign. At the time, Trump was attacking his political opponent, former Secretary of State Hillary Clinton, after some of her staff members took the Fifth Amendment while being questioned by the House Select Committee during their investigation into the 2012 attack on U.S. facilities in Benghazi.
Trump said: “So there are five people taking the Fifth Amendment. Like you see on the mob, right? You see the mob takes the Fifth. If you’re innocent why are you taking the Fifth Amendment?”