Musnad Abu Ya'la al-Mawsili 42:421

And from Abu Sa’id, from the Messenger of Allah, may Allah bless him and grant him peace, who said: Musa said: "O my Lord, teach me something by which I may remember You and call upon You." He said: "Say, O Musa, 'There is no god but Allah' (La ilaha illa Allah)." He said: "All of Your servants say this." He said: "Say, 'There is no god but Allah'." He said: "There is no god but You. I only want something that You single me out with." He said: "O Musa, if the seven heavens and those who inhabit them, other than Me, and the seven earths were in a scale, and 'There is no god but Allah' were in a scale, 'There is no god but Allah' would outweigh them."