Surah Al-A'raf 7:142

۞ وَوَٰعَدْنَا مُوسَىٰ ثَلَـٰثِينَ لَيْلَةً وَأَتْمَمْنَـٰهَا بِعَشْرٍ فَتَمَّ مِيقَـٰتُ رَبِّهِۦٓ أَرْبَعِينَ لَيْلَةً ۚ وَقَالَ مُوسَىٰ لِأَخِيهِ هَـٰرُونَ ٱخْلُفْنِى فِى قَوْمِى وَأَصْلِحْ وَلَا تَتَّبِعْ سَبِيلَ ٱلْمُفْسِدِينَ

And We made an appointment with Moses for thirty nights and perfected them by [the addition of] ten; so the term of his Lord was completed as forty nights. And Moses said to his brother Aaron, "Take my place among my people, do right [by them], and do not follow the way of the corrupters."

Translation: Saheeh International

Tafsir

And We appointed read wa-wā‘adnā or wa-wa‘adnā for Moses thirty nights at the end of which We would speak to him after he has fasted during that period; that was the month of Dhū’l-Qa‘da. He completed the fast. But when it came to an end he disliked the bad odour of his mouth and so cleaned his teeth. …

Excerpt from Tafsir al-Jalalayn, commentary on 7:142, via Quran.com. The reader has the full commentary and 9 more tafsirs.