Eruvin1: 3
הַקּוֹרָה שֶׁאָמְרוּ — רְחָבָה כְּדֵי לְקַבֵּל אָרִיחַ; וְאָרִיחַ חֲצִי לְבֵנָה שֶׁל שְׁלֹשָׁה טְפָחִים. דַּיָהּ לַקּוֹרָה שֶׁתְּהֵא רְחָבָה טֶפַח כְּדֵי לְקַבֵּל אָרִיחַ לְאָרְכּוֹ.
The crossbeam of which they spoke — it is wide [enough] to hold an ariach; an ariach is half a brick measuring three fists. It is sufficient for the crossbeam to be one fist wide to hold an ariach lengthwise.