Tosefta for Berakhot 5:1
אֵין עוֹמְדִין לְהִתְפַּלֵּל אֶלָּא מִתּוֹךְ כֹּבֶד רֹאשׁ. חֲסִידִים הָרִאשׁוֹנִים הָיוּ שׁוֹהִים שָׁעָה אַחַת וּמִתְפַּלְּלִים, כְּדֵי שֶׁיְּכַוְּנוּ אֶת לִבָּם לַמָּקוֹם. אֲפִלּוּ הַמֶּלֶךְ שׁוֹאֵל בִּשְׁלוֹמוֹ, לֹא יְשִׁיבֶנּוּ. וַאֲפִלּוּ נָחָשׁ כָּרוּךְ עַל עֲקֵבוֹ, לֹא יַפְסִיק:
One should not rise to pray except with koved rosh [humility and fear, it being written (Psalms 2:11): "Serve the L-rd with fear"; and this "service" is prayer.] The early pietists would abide one hour [in the place where they came to pray] before beginning to pray in order to direct their thoughts to the L-rd. Even if the king greets him (while he is praying), he may not answer him. [the king of Israel, specifically; but he does interrupt his prayer for a heathen king, lest he kill him.] And even if a snake is wound around his ankle he may not interrupt his prayer. [a snake, specifically, for in most instances they do not bite; but for a scorpion or an adder, things which certainly bite and kill, he does interrupt it.]