Berakhot 24B

וממשמש בבגדו, אבל לא היה מתעטף. וכשהוא מפהק היה מניח ידו על סנטרו.

and if he was stung by a louse, he may feel for it and remove it with his garment, but he would not wrap himself in his prayer shawl if it fell during prayer. And when he would yawn he would place his hand on his chin so that his open mouth would not be visible.

מיתיבי: המשמיע קולו בתפלתו ⁠— הרי זה מקטני אמנה. המגביה קולו בתפלתו הרי זה מנביאי השקר.

The Talmud raises an objection based on a baraita: One who sounds his voice during his Amida prayer is among those of little faith, as he seems to believe that YHWH cannot hear his prayer when it is uttered silently. One who raises his voice during prayer is considered to be among the false prophets, as they too were wont to cry out and shout to their gods.

מגהק ומפהק ⁠— הרי זה מגסי הרוח. המתעטש בתפלתו ⁠— סימן רע לו. ויש אומרים: ניכר שהוא מכוער. הרק בתפלתו ⁠— כאילו רק בפני המלך.

Furthermore, one who belches and yawns while praying is surely among the uncouth. One who sneezes during his prayer, for him it is a bad omen. And some say: It is clear that he is repulsive. Also, one who spits during prayer, it is tantamount to spitting in the face of the king. In light of all this, how could R' Yehuda HaNasi have done all that while praying?

בשלמא מגהק ומפהק לא קשיא: כאן לאונסו, כאן לרצונו. אלא מתעטש אמתעטש קשיא!

The Talmud explains: Granted, with regard to one who belches and yawns, it is not difficult: Here, in the case where R' Yehuda HaNasi did so, it was involuntary and therefore permissible; here, where it is considered uncouth, is in a case where it is deliberate. However, the contradiction between sneezing in the case where R' Yehuda HaNasi did so and sneezing where it is considered a bad omen is difficult.