Two Witnesses

Two Witnesses

דברים יז, ו: עַל-פ ִי ש ְׁנַיִם עֵּדִ ים By the testimony of two witnesses
Baal HaTurim notes a tradition that the letter פ (of the word פי) is written in a form called פי כפולה, a doubled Peh. This indicates that the testimony of the witnesses are acceptable only from their mouths, but not from their writings (for example in a written document).
The name of the letter, פ"ה, reminds one of the word פה, which means mouth. Thus, the doubled Peh emphasizes that the testimony is by the mouth of the witnesses to the exclusion of a written message.