Imam Ahmad bin Hanbal, may Allah have mercy on him, recorded that Ma`diykarib said: "We came to `Abdullah and asked him to recite to us:
طسم
(Ta Sin Mim.) the two hundred. He said, `I do not know it; you should go to someone who learned it from the Messenger of Allah ﷺ Khabbab bin Al-Aratt.' So we went to Khabbab bin Al-Aratt and he recited it to us, may Allah be pleased with him."
بِسْمِ اللَّهِ الرَّحْمَـنِ الرَّحِيمِ
In the Name of Allah, the Most Gracious, the Most Merciful.
The Story of Musa and Fir`awn, and what Allah intended for Their People
We have already discussed the significance of the separate letters.
تِلْكَ ءَايَـتُ الْكِتَـبِ الْمُبِينِ
(These are the Ayat of the manifest Book.) means the Book which is clear and makes plain the true reality of things, and tells us about what happened and what will happen.
Surah Al-Qasas is the very last among the Makki Surahs. It was revealed between Makkah and Juhfah (Rabigh) during the migration. It is mentioned in some narrations that during the journey from Makkah to Madinah when the Holy Prophet ﷺ reached near Juhfah or Rabigh, Jibra'il (علیہ السلام) came to him and enquired whether he was feeling home sick, as he was leaving the place where he was born and spent all his life. To this the Holy Prophet ﷺ replied in the affirmative. Then Jibra'il (علیہ السلام) recited this Surah and gave him the good tiding that ultimately Makkah will fall to him and come under his control. The related verse is as follows:
Surely the one who has enjoined the Qura'n upon you will surely bring you back to the place of return - 28:85
Surah Al-Qasas begins with the story of Sayyidna Musa first briefly then in detail. Firs- half of the Surah contains Sayyidna Musa's (علیہ السلام) episodes with the Pharaoh, and towards the end of the Surah his episode with Qarun is related.
The story of Sayyidna Musa (علیہ السلام) is related extensively in the Qur’ an, at places briefly while at others in detail. In Surah Al-Kahf (Cave) that part of the story is related in which he encountered Al-Al-khadir (علیہ السلام) . Then, in Surah Taha other parts of the story are related in some detail, while some details are related in Surah An-Naml. Now, in Surah Al-Qasas they are being reiterated. In Surah Taha where Allah Ta` ala has said about Musa (علیہ السلام) فَتَنَّاكَ فُتُونًا (And We tested you with a great ordeal - 20:40). Imam Nasa'i and some other commentators have penned down the entire story at that point. The writer has followed suit, and by quoting Ibn Kathir has written the whole story under Surah Taha. All the relevant parts of the story, it's related important issues, and rulings have been discussed in detail under Surahs al-Kahf and Taha. For any reference of an issue or ruling it would be apt to look up there. Now in this Surah commentary will be restricted to brief explanation of the wordings of these verses.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.