As Salaamu Alaykum
Is there any proof that we can't pray for a Muslim's sins/a non-Muslim's sins? A Hadith states that praying for a Muslim's forgiveness is worth one good deed.
And about the reference of doing dua about non-Muslims:
The reference is not only in Ahadith but it is in Qur'an itself too. When Prophet Nooh (PBUH) did dua from Allah SWT about his Son, then Allah SWT forbade him to do so.
Here is the reference from Quran:
011.044 وَقِيلَ يَا أَرْضُ ابْلَعِي مَاءَكِ وَيَا سَمَاءُ أَقْلِعِي وَغِيضَ الْمَاءُ وَقُضِيَ الأمْرُ وَاسْتَوَتْ عَلَى الْجُودِيِّ وَقِيلَ بُعْدًا لِلْقَوْمِ الظَّالِمِينَ
011.045 وَنَادَى نُوحٌ رَبَّهُ فَقَالَ رَبِّ إِنَّ ابْنِي مِنْ أَهْلِي وَإِنَّ وَعْدَكَ الْحَقُّ وَأَنْتَ أَحْكَمُ الْحَاكِمِينَ
011.046 قَالَ يَا نُوحُ إِنَّهُ لَيْسَ مِنْ أَهْلِكَ إِنَّهُ عَمَلٌ غَيْرُ صَالِحٍ فَلا تَسْأَلْنِي مَا لَيْسَ لَكَ بِهِ عِلْمٌ إِنِّي أَعِظُكَ أَنْ تَكُونَ مِنَ الْجَاهِلِينَ
011.047 قَالَ رَبِّ إِنِّي أَعُوذُ بِكَ أَنْ أَسْأَلَكَ مَا لَيْسَ لِي بِهِ عِلْمٌ وَإِلا تَغْفِرْ لِي وَتَرْحَمْنِي أَكُنْ مِنَ الْخَاسِرِينَ
011.048 قِيلَ يَا نُوحُ اهْبِطْ بِسَلامٍ مِنَّا وَبَرَكَاتٍ عَلَيْكَ وَعَلَى أُمَمٍ مِمَّنْ مَعَكَ وَأُمَمٌ سَنُمَتِّعُهُمْ ثُمَّ يَمَسُّهُمْ مِنَّا عَذَابٌ أَلِيمٌ
011.044 Then the word went forth: "O earth! swallow up thy water, and O sky! Withhold (thy rain)!" and the water abated, and the matter was ended. The Ark rested on Mount Judi, and the word went forth: "Away with those who do wrong!"
011.045 And Noah called upon his Lord, and said: "O my Lord! surely my son is of my family! and Thy promise is true, and Thou art the justest of Judges!"
011.046 He said: "O Noah! He is not of thy family: For his conduct is unrighteous. So ask not of Me that of which thou hast no knowledge! I give thee counsel, lest thou act like the ignorant!"
011.047 Noah said: "O my Lord! I do seek refuge with Thee, lest I ask Thee for that of which I have no knowledge. And unless thou forgive me and have Mercy on me, I should indeed be lost!"
011.048 The word came: "O Noah! Come down (from the Ark) with peace from Us, and blessing on thee and on some of the peoples (who will spring) from those with thee: but (there will be other) peoples to whom We shall grant their pleasures (for a time), but in the end will a grievous penalty reach them from Us."
Al-Qur'an, 011.044-048 (Hud [Hud])
WassalaamuAlaikum WRWB,
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