Mishnah 2

קָבְרוּ אֶת הַמֵּת וְחָזְרוּ, אִם יְכוֹלִין לְהַתְחִיל וְלִגְמֹר עַד שֶׁלֹּא יַגִּיעוּ לַשּׁוּרָה – יַתְחִילוּ, וְאִם לָאו – לֹא יַתְחִילוּ. הָעוֹמְדִים בַּשּׁוּרָה: הַפְּנִימִים – פְּטוּרִין, וְהַחִיצוֹנִים – חַיָּבִין.

Those who buried the deceased and have returned; if they are able to start and finish before returning to the line, they should begin [to recite the Shema], but if not, they do not begin. As for those standing in the line; the inner ones are exempt and the outer ones are obligated.