Musnad Ahmad 30:2131

'Affan narrated to us, Hammad—meaning Ibn Salamah—narrated to us, from 'Ata' ibn al-Sa'ib, from al-Agharr, from Abu Hurayrah, from the Prophet (may Allah bless him and grant him peace), in what he narrates from his Lord, the Almighty and Majestic, 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 purer."