Musannaf Ibn Abi Shayba 3:3915

Waki’ narrated to us, he said: Yunus bin Abi Ishaq narrated to us, from Bakr bin Ma’iz, from al-Rabi’ bin Khuthaym, that he said to his mu’adhdhin: "When it is a cloudy day, then delay the Maghrib prayer until the darkness of the night sets in."