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].

وعن ابن عباس رضي الله عنهما أن رسول الله ﷺ قال: "لا يخلون أحدكم بامرأة إلا مع ذي محرم" ((متفق عليه)).