The story of Avraham Avinu's hachnasas orchim and the three malachim is written in the past tense: "He took... He went... He hurried... etc." The exception is at the words (18:8), ויאכלו העץ תחת עליהם עומד והוא, "[Avraham] stands above them..." These words are written in the present tense because, until today, when Yidden do hachnasas orchim, Avraham stands there and partakes in the mitzvah. As Rebbe Moshe of Kobrin zt'l teaches, when a Yid performs hachnasas orchim, Avraham comes and participates in the mitzvah.
Reb Chaim Brisker zt'l was once very ill as a child, and the doctors lost hope. Rebbe Moshe Kobriner was visiting Brisk at the time, and he stayed in the home of Reb Chaim's grandfather (Reb Itche Efron z'l, the father-in-law of the Beis HaLevi). The grandfather asked the Rebbe, "What will be with my Chaimke? He is so ill."
Rebbe Moshe Kobriner told him the vort, עומד אברהם, indicating that Avraham is present at every hachnasas orchim. He added, "Chazal (Bava Basra 16:) say, 'A precious stone hung from Avraham Avinu's neck. Whoever saw the stone was healed.' Since you are hosting me and performing hachnasas orchim, Avraham Avinu is here, in this home. And Avraham wears the gem that brings refuah. Your Chaimke will surely have a refuah sheleimah." And indeed, he did.
(This story is written in Machsheves Nachum, by Reb Nachum Mikontikoziva-Chaifa. He adds that he told this story to Reb Chaim Brisker, and Reb Chaim said he also knows this story. He heard it from his father, the Beis HaLevi.)
Once, Rebbe Moshe Kobriner was a guest in the home of Rebbe Aharon Karliner zt'l, and Reb Aharon told him that his daughter was ill. Rebbe Moshe Kobriner said the sick girl, "Your father is performing hachnasas orchim. This means right now, he has the stone of healing hanging on his neck. Look at your father, and you will be healed."
Rebbe Moshe Mordechai of Lelov zt'l once spoke to his chassidim about a certain Yemenite tzaddik who fed hundreds of refugees who lost their homes in World War I. One of the chassidim said, "I heard that this tzaddik was a chavrusah with one of the Kabbalah giants." He was implying that in addition to his chesed, this tzaddik was a scholar in the hidden parts of the Torah. The Rebbe replied, "Why are you mentioning that? Right now, we are discussing that he fed hundreds of homeless Yidden..." As if to say the merit of feeding Yidden is so great, it isn't necessary to add anything else.
