Ezekiel 44:22

וְאַלְמָנָה וּגְרוּשָׁה לֹא יִקְחוּ לָהֶם לְנָשִׁים כִּי אִם בְּתוּלֹת מִזֶּרַע בֵּית יִשְׂרָאֵל וְהָאַלְמָנָה אֲשֶׁר תִּהְיֶה אַלְמָנָה מִכֹּהֵן יִקָּחוּ

They shall not take a widow or a divorcee for their wives, only virgins from the family of the house of Israel or a widow who is the widow of a priest.

This verse is discussing the third Temple era and it appears at odds with our current practice, that a kohein may marry a widow, though the Kohein Gadol (High Priest) may not. The Radak offers two explanations for this verse. The first suggests an increased level of sanctity in the third Temple era, limiting even regular kohanim to the widows of kohanim. The second – preferred by most other commentators and the Talmud (Kiddushin 78b)- understands the first part of the verse to be addressing only High Priests and the second part to say that some kohanim – that is, regular kohanim – may marry widows (in other words, reinforcing the status quo).