Musa (AS) had a special relationship with Allah and so He asked his Lord for a special dua. He wanted a dua that was better and more pleasing to Allah than any other supplication.
Allah told him to recite, “La illaha illallah.” Musa was disappointed, as this is the declaration of faith, something which every other Prophet (AS) and Believer had already been taught.
However! This dua is so powerful that when placed on a scale of deeds, it weighs heavier than the earth and 7 heavens combined. SubhanAllah!