{Then We gave Moses the Scripture, completing [the favor] upon the one who did good} — Qatadah said: Whoever does good in this world, the favor is completed for him in the Hereafter. {And as a detailed explanation} — meaning: an elucidation {of all things} concerning the lawful and the unlawful, and guidance and misguidance.
﴿ثُمَّ آتَيْنَا مُوسَى الْكِتَابَ تَمَامًا على الَّذِي أحسن﴾ قَالَ قَتَادَةُ:
مَنْ أَحْسَنَ فِي الدُّنْيَا تَمَّتْ عَلَيْهِ النِّعْمَةُ فِي الْآخِرَة ﴿وتفصيلا﴾ يَعْنِي: تبييناً ﴿لكل شيءٍ﴾ مِنَ الْحَلَالِ وَالْحَرَامِ، وَالْهُدَى وَالضَّلَالِ.