Commentary for Berakhot 51:2

כמה ירחיק מהן ומן הצואה ארבע אמות: אמר רבא אמר רב סחורה אמר רב הונא לא שנו אלא לאחוריו אבל לפניו מרחיק מלא עיניו וכן לתפלה

To what distance must he remove himself from it and excrement [if he wishes to read the Shema'] ? Four cubits. Raba said in the name of Rab Sahorah in the name of Rab Huna : This teaching applies only to [excrement which is behind him ; but if it is in front of him, he must remove himself as far as the eye can see it. Similarly is it with the Tefillah.

Abraham Cohen Footnotes to the English Translation of Masechet Berakhot