Musar for Bava Metzia 172:16

Now to the triple significance of the word "Moriah." When Abraham saw the three men approach he was in doubt as to whether they were angels or just Arabs; it was the hottest part of the day and normal people stayed indoors; there was no escaping from G–d's heat. He therefore concluded that – if they were angels – these three men must be angels dressed up as human beings possessing a body impervious to the emanations of heat. On the other hand, if they were angels, why would they bow down to the dust of their feet? They therefore had to be Arabs bowing to the dust of their feet under the mistaken impression that the sun causes the accumulation of dust, and that therefore bowing to the dust was a form of sun-worship. Considerations such as these went through Abraham's mind. He was convinced that they could only be either angels or sun worshipers as no one else would risk being out in such a heat.