Isaiah 20:3
And the Lord said, Like as my servant Isaiah hath walked naked and barefoot three years for a sign and wonder upon Egypt and upon Ethiopia;
Cross-reference
Isaiah 8:18
Parallel
Isaiah 8:18 explicitly calls Isaiah and his children signs and portents, just as he is called a sign here.
Ezekiel 24:24
Parallel
Ezekiel 24:24 states that Ezekiel himself is a sign, exactly as Isaiah is called a sign in Isaiah 20:3.
Zechariah 3:8
Parallel
Zechariah 3:8 calls Joshua and his friends 'men who are a sign' — parallel to Isaiah being a sign.