ויהי יושב פתח האוהל כחום היום (וירא יח:א)

The nature of people is to be warmed up to a mitzvah at the onset of performing it; with time, his enthusiasm somewhat cools off.  Avraham Avinu, though, was the opposite.  Even though three days had passed in which he experienced post-Bris pain, his approach to the mitzvah of Hachnosas Orchim didn’t wane – he performed it with enthusiasm as it says “כחום היום” – with the same heat and desire that he performed the mitzvah of Bris Milah.

(Tiferes Shlomo)