Musannaf Ibn Abi Shayba 33:387

Yazid ibn Harun narrated to us, he said: Uyaynah ibn Abd al-Rahman informed us, from Ayyub ibn Musa, from Abu Hazim, from Sa’id ibn al-Musayyib, who said: "Indeed, the mosques have pegs among the servants of Allah, whose companions are the angels. If they are missing, they (the angels) ask about them; if they are ill, they visit them, and if they are in need, they assist them."