I read that in Islam, it is haram to mistreat slaves. Instead they must be treated with care.
Could you provide some Quran verses or Hadith supporting this (with a brief explanation if necessary)?
Prophet Muhammad said:
" الاكل مع الخدام من التواضع، فمن أكل معهم اشتاقت إليه الجنة "
Eating with slaves/servants is from humility، and he who eats with them makes the heaven/pradise miss him.
As missing him because he's showing modesty by eating with them, and not treating them horribly.