An amazing story:
The city of Hevron, many years ago, did not always have ten Jewish adults, but for the Jewish Sabbath and for the holidays they always managed a minyan.
One, on the day before Yom Kippur, a count was made, and it was discovered that some people had left for Jerusalem that day, and there were only 9 adults for the holiday. For the first time in history, there would be no minyan in the synagogue on Yom Kippur.
They waited until the last minute, hoping that perhaps a nearby Jewish farmer would show up, but no one came. Many Jews preferred to spend the day in Jerusalem which was only a few hours away. It offered them an opportunity to spend part of the day praying a the Kosel Maarovi, (the wailing wall). The remaining nine people were very upset at not having a minyan, and many actually cried.
Just as the sun was about to set, which meant that the holy day was about to start, they noticed a person approaching from a distance. He was old, with a long silver-white beard. They all rushed to meet the new arrival, quickly took him to the nearest residence, and serviced him a hurried meal. However he did not eat the meal, claiming that he had already eaten the seudah hamafsekes, (the last meal that one partakes of before the start of the holy day). The old man thanked them and blessed them. With joy in their hearts, they started for the nearby synagogue. They spent all that evening and the next day praying to the L-rd and thanking Him for providing them with a tenth for the minyan.
When they finished the services for the next evening, each man was anxious to have the guest in his home for the evening meal. They seldom had guests and wanted to observe the great mitzvah of hachnasas orchim. They had no alternative but to cast lots to determine who would be the lucky one to have the guest at his house. This was soon done, and the lucky person to win the mitzvah was the cantor who was known to be a great tzaddik.
The cantor rushed home with the guest following him. As they reached the door, the cantor stepped aside to give the important guest the honor of entering the house first. To his great astonishment he discovered that the guest, who was walking behind him just a minute before, was not there. The whole congregation started to look for the guest. They searched the yard and the street, but it seemed that he had disappeared. They were all dismayed that the guest had left without partaking of their food. That night the old man appeared for the cantor in a dream and revealed to him that he was father Avraham who was laid to rest in the me'aras hamachpela, the double cave located nearby. He had come to complete the minyan for Yom Kippur.
The congregation was very happy, blessed G-d for the great miracle, and thanked Him for His kindness to them.
From: Torah thoughts Book Three, by Leo Gartenberg published in 1966.