עִבְד֣וּ אֶת־יְהוָ֣ה בְּיִרְאָ֑ה וְ֝גִ֗ילוּ בִּרְעָדָֽה׃
דינט גאָט מיט מורא, און שױדערט אין ציטערניש.
Mishnah Berakhot
One should not stand up to say Tefillah except in a reverent state of mind. The pious men of old used to wait an hour before praying in order that they might direct their thoughts to God. Even if a king greets him [while praying] he should not answer him: even if a snake is wound round his heel he should not stop.