Musnad Ahmad 30:1448

Bahz and ‘Affan narrated to us, they said: Hammam narrated to us, Qatadah narrated to us, from An-Nadr ibn Anas, from Bashir ibn Nahik, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever has two wives and inclines toward one of them over the other, he will come on the Day of Resurrection with one of his sides drooping."