The haftarah for parshas Vayeira opens with the story in Melachim II chapter 4 of the widow of the prophet Ovadyah (as Rashi brings on 4:1) and the miracle Elisha performs for her.
There is a famous explanation from Midrash Tanchuma in Mishpatim 9 that Ovadyah was hiding a group of prophets in two caves during the time king Achav was trying to kill out all of the prophets of Hashem, so Ovadyah bought for these people bread, water and oil for light. Since it was a pikuach nefesh, he borrowed from Achav, who would only lend at interest.
A couple of years ago, I learned that the miracle was done particularly because Ovadyah provided oil for these people, -- in other words, bread and water were basic necessities, but oil was only needed for comfort since they could have learned by heart. So for providing that which he was not as obligated to provide, Ovadyah merited a miracle which is described in Melachim II 4 at length.
I would like to know the source for this idea, as I am sure I heard it from some rav or read in a good sefer, but cannot recall the source for that idea.
Thank you very much for your help.