סימן קעט סעיף ג
אָסוּר לְהַלְווֹת בְּלֹא עֵדִים וַאֲפִלּוּ לְתַלְמִיד חָכָם, אֶלָּא אִם כֵּן מַלְוֵהוּ עַל הַמַּשְׁכּוֹן. וְהַמַּלְוֵה בִּשְׁטָר, מְשֻׁבָּח יוֹתֵר.
It is forbidden to lend money without witnesses,7Maseches Baba Metzia 75b. Shulchan Aruch 70:1. The reason is because you are placing a stumbling block in front of a person, who might be tempted to deny that you lent him. Also you may cause yourself to be cursed, for people may suspect you of lodging a false claim. Today people are not careful about this, and the Peri Yitzchok Vol. 1 Responsa 48 explains why. However, he, too, confirms that you should be careful to follow this halacha, as brought in Rif, Rambam, Rosh, Shulchan Aruch etc. without question. not even to a Torah scholar,8Because he is engrossed in his Torah learning, he might forget that he borrowed the money. unless you are lending against a security. To lend money with legal documentation is the best course to take.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)