Sahih al-Bukhari 4213

Narrated Anas:The Prophet (ﷺ) stayed for three rights between Khaibar and Medina and was married to Safiya. I invited the Muslim to h s marriage banquet and there wa neither meat nor bread in that banquet but the Prophet ordered Bilal to spread the leather mats on which dates, dried yogurt and butter were put. The Muslims said amongst themselves, "Will she (i.e. Safiya) be one of the mothers of the believers, (i.e. one of the wives of the Prophet (ﷺ) ) or just (a lady captive) of what his right-hand possesses" Some of them said, "If the Prophet (ﷺ) makes her observe the veil, then she will be one of the mothers of the believers (i.e. one of the Prophet's wives), and if he does not make her observe the veil, then she will be his lady slave." So when he departed, he made a place for her behind him (on his and made her observe the veil.