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." (ID)
English reference (ID): Vol. 3, Book 9, Hadith 1969 (ID)
Arabic reference (ID): Book 9, Hadith 2045 (ID)