Mishnah for Berakhot 3:1
מִי שֶׁמֵּתוֹ מוּטָל לְפָנָיו, פָּטוּר מִקְּרִיאַת שְׁמַע, מִן הַתְּפִלָּה וּמִן הַתְּפִלִּין. נוֹשְׂאֵי הַמִּטָּה וְחִלּוּפֵיהֶן וְחִלּוּפֵי חִלּוּפֵיהֶן, אֶת שֶׁלִּפְנֵי הַמִּטָּה וְאֶת שֶׁלְּאַחַר הַמִּטָּה, אֶת שֶׁלַּמִּטָּה צֹרֶךְ בָּהֶן פְּטוּרִים, וְאֶת שֶׁאֵין לַמִּטָּה צֹרֶךְ בָּהֶן חַיָּבִין. אֵלּוּ וָאֵלּוּ פְּטוּרִים מִן הַתְּפִלָּה:
One whose dead one [(one of his kin that he is obliged to mourn, whom it is incumbent upon him to bury)] is lying before him, is exempt from the recital of the Shema [being preoccupied with a mitzvah], from tefillah, and from tefillin. As to the litter bearers, their replacements, [it being the practice to alternate, all wishing to partake in the mitzvah], and the latter's replacements, those before the litter [those set to carry it when it reaches them], and those behind it [that is, both those before it and those behind it] — those whose services are required [for carrying the litter], are exempt; and those whose services are not required [such as those who go only to accompany the deceased to honor him] are obligated. And both are exempt from tefillah, [it not being Scripturally mandated as is the recital of the Shema. And some say because it requires a greater measure of concentration.]