Ibn Abbas (May Allah be pleased with them) said:
The Messenger of Allah (ﷺ) said, "No one of you should meet a woman in privacy unless she is accompanied by a Mahram (i.e., a relative within the prohibited degrees)."[Al-Bukhari and Muslim]. (RU)
App reference (RU): Book 18, Hadith 119 (RU)
Arabic/English book reference (RU): Book 18, Hadith 1629 (RU)