Berakhot 47a
ברכות מ״ז א — ד"ה תָּנוּ רַבָּנַן
Berakhot 47a:10
The Talmud discusses the laws of responding to blessings, including the requirement to hear the one reciting and the circumstances in which one may answer Amen. This is a foundational source for the question of hearing a berakhah.
תָּנוּ רַבָּנַן: אֵין עוֹנִין לֹא ״אָמֵן״ חֲטוּפָה, וְלֹא ״אָמֵן״ קְטוּפָה, וְלֹא ״אָמֵן״ יְתוֹמָה, וְלֹא יִזְרוֹק בְּרָכָה מִפִּיו.
With regard to answering amen, the Sages taught: One should not respond with an abbreviated [ḥatufa] amen, in which the first syllable is not properly enunciated, and a truncated [ketufa] amen, in which the second syllable is not properly enunciated, and an orphaned [yetoma] amen, in which the respondent is unaware of the blessing to which he is responding. Similarly, one should not quickly and indifferently discharge a blessing from his mouth.