KJV Hebrews 13:2 says,
"Be not forgetful to entertain strangers: for thereby some have entertained angels unawares."
The tendency is 'gloss past' this passage; it's a reminder to be hospitable, but in every angelic encounter in Scripture, either the angel is identified, or the fact that there was an angelic visitation was identified.
Is Paul merely citing a Jewish proverb, or is there some basis for this saying?