✅ Answer: Note the following hadiths:

1 (عن علي بن جعفر عن موسي بن جعفر قال) سَأَلْتُهُ عَنِ اَلرَّجُلِ يَقْرَأُ سُورَةً وَاحِدَةً فِي اَلرَّكْعَتَيْنِ مِنَ اَلْفَرِيضَةِ وَ هُوَ يُحْسِنُ غَيْرَهَا، فَإِنْ فَعَلَ فَمَا عَلَيْهِ؟ قَالَ:

إِذَا أَحْسَنَ غَيْرَهَا فَلاَ يَفْعَلْ، وَ إِنْ لَمْ يُحْسِنْ غَيْرَهَا فَلاَ بَأْسَ، وَ إِنْ فَعَلَ فَلاَ شَيْءَ عَلَيْهِ، وَ لَكِنْ لاَ يَعُودُ

📚 Qurb al-Isnad, p. 206

Ali ibn Jafar said: I asked Imam Kazim (peace be upon him) about a person who recites a single surah in both rak'ahs of an obligatory prayer while he knows others besides it—what is his duty if he does this? The Imam said: If he knows a surah other than that one, he should not do this; but if he does not know any other, there is no problem. If he has already done it, there is nothing upon him (no penalty), but he should not repeat this action!

2 عَنْ صَفْوَانَ الْجَمَّالِ قَالَ سَمِعْتُ أَبَا عَبْدِ اللَّهِ یَقُولُ قُلْ هُوَ اللَّهُ أَحَدٌ تُجْزِی فِی خَمْسِینَ صَلَاةً.

📚 Al-Tahdhib, Vol. 2, p. 96

Safwan al-Jammal said: I heard Imam Sadiq (peace be upon him) saying: Surah 'Qul Huwa Allahu Ahad' (Al-Ikhlas) is sufficient in all fifty rak'ahs (daily obligatory and supererogatory prayers)!

3 عَنْ عُبَیْدِ بْنِ زُرَارَةَ وَ أَبِی إِسْحَاقَ ثَعْلَبَةَ عَنْ زُرَارَةَ قَالَ: قُلْتُ لِأَبِی جَعْفَرٍ أُصَلِّی بِقُلْ هُوَ اللَّهُ أَحَدٌ فَقَالَ نَعَمْ قَدْ صَلَّی رَسُولُ اللَّهِ فِی کِلْتَا الرَّکْعَتَیْنِ بِقُلْ هُوَ اللَّهُ أَحَدٌ لَمْ یُصَلِّ قَبْلَهَا وَ لَا بَعْدَهَا بِقُلْ هُوَ اللَّهُ أَحَدٌ أَتَمَّ مِنْهَا

📚 Al-Tahdhib, Vol. 2, p. 96

Zurarah said: I said to Imam Baqir (peace be upon him): May I pray using Surah 'Qul Huwa Allahu Ahad'? He said: Yes! Indeed, the Messenger of Allah (peace be upon him and his family) prayed both rak'ahs with Surah 'Qul Huwa Allahu Ahad', and he never recited 'Qul Huwa Allahu Ahad' more perfectly than he did before or after that!

4 عَنْ صَفْوَانَ الْجَمَّالِ قَالَ سَمِعْتُ أَبَا عَبْدِ اللَّهِ یَقُولُ صَلَاةُ الْأَوَّابِینَ الْخَمْسُونَ کُلُّهَا بِقُلْ هُوَ اللَّهُ أَحَدٌ.

📚 Al-Kafi, Vol. 3, p. 314

Safwan al-Jammal said: I heard Imam Sadiq (peace be upon him) saying: The prayer of the 'Awwabin' (the oft-returning) is fifty rak'ahs, and all are recited with Surah 'Qul Huwa Allahu Ahad'!

5 عَنْ عِمْرَانَ بْنِ الْحُصَیْنِ أَنَّ النَّبِیَّ بَعَثَ سَرِیَّةً وَ اسْتَعْمَلَ عَلَیْهَا عَلِیّاً فَلَمَّا رَجَعُوا سَأَلَهُمْ فَقَالُوا کُلٌّ خَیْرٌ غَیْرَ أَنَّهُ قَرَأَ بِنَا فِی کُلِّ الصَّلَوَاتِ قُلْ هُوَ اللَّهُ أَحَدٌ- فَقَالَ یَا عَلِیُّ لِمَ فَعَلْتَ هَذَا فَقَالَ لِحُبِّی بِقُلْ هُوَ اللَّهُ أَحَدٌ فَقَالَ النَّبِیُّ مَا أَحْبَبْتَهَا حَتَّی أَحَبَّکَ اللَّهُ.

📚 Al-Tawhid, p. 94

Imran ibn Husayn said: The Messenger of Allah (peace be upon him and his family) sent an army and appointed Amir al-Mu'minin (peace be upon him) as its commander. When they returned, he questioned them, and they said: Everything was good, except that Amir al-Mu'minin (peace be upon him) recited Surah 'Qul Huwa Allahu Ahad' for us in all the (congregational) prayers!

So the Prophet said: O Ali! Why did you do this? He replied: Because of the love I have for Surah 'Qul Huwa Allahu Ahad'! Then the Messenger of Allah (peace be upon him and his family) said: You did not take it as a friend until Allah took you as a friend (beloved)!

✅ Based on the above hadiths and others, repeating the same surah in the second rak'ah that was recited in the first rak'ah of an obligatory prayer is discouraged (Makruh), unless the worshiper only knows that one surah and does not know how to recite any other.

However, Surah 'Qul Huwa Allahu Ahad' is an exception to this discouragement, and the worshiper can recite Surah 'Qul Huwa Allahu Ahad' after Al-Hamd in all daily obligatory and supererogatory prayers.

✅ The proof in answering jurisprudential questions is solely the relevant verses, hadiths, and the luminous words of the Infallibles; the concluding explanations are only to assist understanding and are subject to critique.

اللَّهُمَّ صَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْر

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