Related su Esodo 32:21

One who leads the responsive recitation of the Shema, or blesses over fruit or over matzah, behold, he should not answer "amen" after himself. If he answered ["amen" after his own prayer], behold, this is how ignoramuses act. We do not answer with either an "orphaned amen" (i.e., where one does not know to what blessing he is saying "amen"), or a "truncated [amen]" (i.e., where one does not pronounce the entire word). Ben Azzai says, one who answers with an "orphaned amen" -- his children will be orphaned; with a "truncated [amen]," his days will be truncated. But whoever prolongs [his amen], his days and years will be prolonged.