Abu Sa'id al-Khudri reported God’s messenger as saying that Moses asked his Lord to teach him something with which to make mention of Him or to supplicate Him, and was told to say, “There is no god but God.” He replied to his Lord that all His servants said this, but he wanted something particularly for himself, and He said, “Moses, were the seven heavens and their inhabitants apart from me and the seven earths put in one side of a balance and ‘There is no god but God’ in the other, ‘There is no god but God’ would outweigh them.”
It is transmitted in Shar as-sunna.