Musannaf Ibn Abi Shayba 17:1285

Hafs narrated to us, from Abu Ishaq, from Amr ibn Maymun, who said: When Allah raised Musa as a confidant, he saw a man clinging to the Throne. He said, "O Lord, who is this?" He said, "A righteous servant from among My servants. If you wish, I will inform you of his deed." He said, "O Lord, inform me." He said, "He used to not walk with slander."