What is the ruling on congregants shaking hands with the Imam who is delivering the sermon?

You can point to the person and greet them with your hand without speaking during the greeting while the sermon is ongoing, because the Messenger of Allah, peace and blessings be upon him, commanded listening attentively and considered speaking or touching pebbles as vain during the sermon. It is incumbent upon the believer to listen attentively and with humility. One may pray two rak'ahs for greeting the mosque upon entering while the preacher is delivering the sermon, and one may praise Allah inwardly if they sneeze, without raising their voice.