It was narrated from Abu Hurairah:
that the Messenger of Allah said: "Whoever has two wives and favors one of them over the other, he will come on the Day of Resurrection with one of his sides leaning." (FR)
English reference (FR): Vol. 3, Book 9, Hadith 1969 (FR)
Arabic reference (FR): Book 9, Hadith 2045 (FR)