Surah Al-A'raf, Ayah 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.'

And We appointed for Musa thirty nights and added ten, and he completed the term, appointed by his Lord, of forty nights. And Musa said to his brother Harun: "Replace me among my people, act in the right way and follow not the way of the mischief-makers. (142)

Musa fasts and worships Allah for Forty Days

Allah reminds the Children of Israel of the guidance that He sent to them by speaking directly to Musa and revealing the Tawrah to him. In it, was their law and the details of their legislation. Allah stated here that He appointed thirty nights for Musa. The scholars of Tafsir said that Musa fasted this period, and when they ended, Musa cleaned his teeth with a twig. Allah commanded him to complete the term adding ten more days, making the total forty. When the appointed term finished, Musa was about to return to Mount Tur, as Allah said,

يَا بَنِي إِسْرَائِيلَ قَدْ أَنجَيْنَاكُم مِّنْ عَدُوِّكُمْ وَوَاعَدْنَاكُمْ جَانِبَ الطُّورِ الْأَيْمَنَ

(O Children of Israel! We delivered you from your enemy, and We made a covenant with you on the right side of the Mount)[20:80].

Musa left his brother Harun with the Children of Israel and commanded him to use wisdom and refrain from mischief. This was only a reminder, for Harun was an honorable and noble Prophet who had grace and exalted standard with Allah, may Allah's peace and blessings be upon him and the rest of the Prophets.