Musannaf Ibn Abi Shayba 3:2889

Waki’ narrated to us, from Abdullah bin Sa’id, who said: I heard Sa’id bin al-Musayyib, and a man asked him about a man who said "Yarhamuk Allah" (may Allah have mercy on you) to a man while the Imam was delivering the sermon: Is it void? He said: No, but he should not do it again.