Al-A'raf · 7:142

142. And Allah appointed for Moses a place of meeting of thirty nights in order to speak with him, and then He perfected it by adding ten more, so that there were forty nights all together. Moses told his brother Aaron, when he was leaving for this retreat alone with His Lord: 'O Aaron, take my place as leader among my people, and do what is right, governing them wisely and treating them kindly, and not following the path of those who cause corruption, nor committing disobedience, nor helping the wrongdoers.'