Start of Parshat Vayeira - Genesis Chapter 18 - Verses 1 & 2. Hashem comes to visit Avraham Avinu after his bris. Avraham is sitting outside his tent. From this we learn about bikkur holim (visiting the sick) & the mitzvah of hachnasat orchim (welcoming guests). In order to do true chesed, and therefore emulate Hashem, there has to be 2 steps. One, a bitul ha yesh (self nullification) & two, you must truly see the other person's needs.