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,
يبَنِى إِسْرَءِيلَ قَدْ أَنجَيْنَـكُمْ مِّنْ عَدُوِّكُمْ وَوَاعَدْنَـكُمْ جَانِبَ الطُّورِ الاٌّيْمَنَ
(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.
Tafsir Ibn Kathir (abridged, English).