Musannaf Ibn Abi Shayba 3:2545

Waki’ narrated to us, he said: Sufyan narrated to us, from Isma’il bin Umayyah (or ‘Ulayyah), from al-Mughirah bin Hakim, from Sa’id bin al-Musayyib, that he was asked about sleeping in the mosque. He said: "Where did the People of the Suffah—meaning—sleep? [They slept] in it."