אורח חיים ק״הOrach Chayim 105

Se’if 1א׳

הַמִּתְפַּלֵּל שְׁתֵּי תְּפִלּוֹת, זוֹ אַחַר זוֹ, צָרִיךְ לְהַמְתִּין בֵּין זוֹ לָזוֹ כְּדֵי הִלּוּךְ ד' אַמּוֹת, כְּדֵי שֶׁתְּהֵא דַּעְתּוֹ מְיֻשֶּׁבֶת לְהִתְפַּלֵּל בִּלְשׁוֹן תְּחִנָּה.

The Law Of One Who Prays Two [Amidah] Prayers. Containing 1 Se'if: One who prays two [Amidah] prayers, one after the other, must wait between one and the other [for the time it takes] to walk 4 amot, so that one's understanding may be settled, [in order] to pray with the language of supplication.