Musannaf Ibn Abi Shayba 21:343

Yahya ibn Adam narrated to us, from Mufaddal, from Mansur, from Mujahid, from Abu Bakr ibn Abd al-Rahman ibn Hisham, from Ka’b, who said: Moses said, "O Lord, guide me to a deed that, if I perform it, would be gratitude to You for what You have chosen me for." He said, "O Moses, say: 'There is no god but Allah, alone, having no partner; to Him belongs the dominion, and to Him belongs the praise, and He is capable of all things.'" He said: Moses wanted a deed that was more strenuous for his body than what he had been commanded with. He said: So He said to him, "O Moses, if the seven heavens and the seven earths were placed in a scale, and 'There is no god but Allah' were placed in a scale, it would outweigh them."