Musannaf Ibn Abi Shayba 21:359

Affan narrated to us, Hammad bin Salamah narrated to us, from Ata’ bin al-Sa’ib, from al-Agharr Abu Muslim, from Abu Hurairah, from the Prophet (may Allah bless him and grant him peace), regarding what he narrates from his Lord, who said: "Whoever remembers Me within himself, I remember him within Myself. And whoever remembers Me in a gathering of people, I remember him in a gathering better than them and more numerous."