Kabbalah zu Schemot 34:28

We have learned that when Moses went up, "he was there with Hashem forty days and forty nights; he did neither eat bread, nor drink water" (Shemot 34:28) in order not to deviate from the custom of the place he went to. And of the angels, when they descended, it is written, "and he stood by them under the tree, and they ate" (Beresheet 18:8). Here also, the descending angel could not struggle with Jacob unless clothed in a body, as is done in this world. Therefore Jacob wrestled with him the whole night. But if he were not clothed (in human form), Jacob could not have wrestled with him.