Musnad Ahmad 30:2301

Qutaybah narrated to us, he said: Ibn Lahi'ah narrated to me, from Darraj, from Ibn Hujayrah, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, the mosques have pegs whose companions are the angels. If they are absent, they miss them; if they are ill, they visit them; and if they are in need, they assist them."