Musnad Abu Ya'la al-Mawsili 138:17

Abu Khaythama narrated to us, Jarir narrated to us, from Layth ibn Abi Sulaym, from Bishr, from Anas ibn Malik, who attributed it to the Prophet (may Allah bless him and grant him peace) regarding His saying: "So by your Lord, We will surely question them all about what they used to do" (Al-Hijr: 92-93). He said: "About 'La ilaha illa Allah' (There is no god but Allah)."