The concession, in the text
Four things sitting together. The Qur'an ties obligation to capacity, and ties it again to what a person has actually been given. It states in the context of the fasting rules that the intention behind them is ease and not hardship. The Prophet, peace be upon him, named pregnant and breastfeeding women alongside the traveller as people for whom the fast is lifted. The report from Umm Salamah is where the forty day figure for nifas comes from. And when a woman asked Aisha whether missed prayers are made up, the answer was that they were not required to make them up.
That last report is about menstruation and the chapter it sits under says so. The extension of the same principle to nifas is the established position across the schools, and the extension itself is a scholar's ruling rather than ours to make.
Qur’an and Sunnah
شَهۡرُ رَمَضَانَ ٱلَّذِيٓ أُنزِلَ فِيهِ ٱلۡقُرۡءَانُ هُدࣰ ى لِّلنَّاسِ وَبَيِّنَٰتࣲ مِّنَ ٱلۡهُدَىٰ وَٱلۡفُرۡقَانِۚ فَمَن شَهِدَ مِنكُمُ ٱلشَّهۡرَ فَلۡيَصُمۡهُۖ وَمَن كَانَ مَرِيضًا أَوۡ عَلَىٰ سَفَرࣲ فَعِدَّةࣱ مِّنۡ أَيَّامٍ أُخَرَۗ يُرِيدُ ٱللَّهُ بِكُمُ ٱلۡيُسۡرَ وَلَا يُرِيدُ بِكُمُ ٱلۡعُسۡرَ وَلِتُكۡمِلُواْ ٱلۡعِدَّةَ وَلِتُكَبِّرُواْ ٱللَّهَ عَلَىٰ مَا هَدَىٰكُمۡ وَلَعَلَّكُمۡ تَشۡكُرُونَ
It was in the month of Ramadan that the Quran was revealed as guidance for mankind, clear messages giving guidance and distinguishing between right and wrong. So any one of you who is present that month should fast, and anyone who is ill or on a journey should make up for the lost days by fasting on other days later. God wants ease for you, not hardship. He wants you to complete the prescribed period and to glorify Him for having guided you, so that you may be thankful
Qur’an 2:185
لَا يُكَلِّفُ ٱللَّهُ نَفۡسًا إِلَّا وُسۡعَهَاۚ لَهَا مَا كَسَبَتۡ وَعَلَيۡهَا مَا ٱكۡتَسَبَتۡۗ رَبَّنَا لَا تُؤَاخِذۡنَآ إِن نَّسِينَآ أَوۡ أَخۡطَأۡنَاۚ رَبَّنَا وَلَا تَحۡمِلۡ عَلَيۡنَآ إِصۡرࣰ ا كَمَا حَمَلۡتَهُۥ عَلَى ٱلَّذِينَ مِن قَبۡلِنَاۚ رَبَّنَا وَلَا تُحَمِّلۡنَا مَا لَا طَاقَةَ لَنَا بِهِۦۖ وَٱعۡفُ عَنَّا وَٱغۡفِرۡ لَنَا وَٱرۡحَمۡنَآۚ أَنتَ مَوۡلَىٰنَا فَٱنصُرۡنَا عَلَى ٱلۡقَوۡمِ ٱلۡكَٰفِرِينَ
God does not burden any soul with more than it can bear: each gains whatever good it has done, and suffers its bad- ‘ Lord, do not take us to task if we forget or make mistakes. Lord, do not burden us as You burdened those before us. Lord, do not burden us with more than we have strength to bear. Pardon us, forgive us, and have mercy on us. You are our Protector, so help us against the disbelievers.’
Qur’an 2:286
لِيُنفِقۡ ذُو سَعَةࣲ مِّن سَعَتِهِۦۖ وَمَن قُدِرَ عَلَيۡهِ رِزۡقُهُۥ فَلۡيُنفِقۡ مِمَّآ ءَاتَىٰهُ ٱللَّهُۚ لَا يُكَلِّفُ ٱللَّهُ نَفۡسًا إِلَّا مَآ ءَاتَىٰهَاۚ سَيَجۡعَلُ ٱللَّهُ بَعۡدَ عُسۡرࣲ يُسۡرࣰ ا
and let the wealthy man spend according to his wealth. But let him whose provision is restricted spend according to what God has given him: God does not burden any soul with more than He has given it- after hardship, God will bring ease
Qur’an 65:7
It was narrated from Anas bin Malik that:he came to Prophet in Al-Madinah when he was eating breakfast. The Prophet said to him: "Come and eat the breakfast." He said: "I am fasting." The Prophet said to him: "Allah, the mighty and sublime, has waived fasting and half of the prayer for the traveler and for pregnant and breastfeeding women
Sunan an-Nasa'i 2315graded sound, albanifull report
Mu'adha reported:A woman asked 'A'isha: Should one amongst us complete prayers abandoned during the period of menses? 'A'isha said: Are you a Haruriya? When any one of us during the time of the Messenger of Allah (ﷺ) was in her menses (and abandoned prayer) she was not required to complete them
Sahih Muslim 335graded sound, sahihaynfull report
Narrated Umm Salamah, Ummul Mu'minin: The woman having bleeding after delivery (puerperal haemorrhage) would refrain (from prayer) for forty days or forty nights; and we would anoint our faces with an aromatic herb called wars to remove dark spots
Sunan Abi Dawud 311graded sound, albanifull report
Translation: M.A.S. Abdel Haleem, The Qur'an, Oxford University Press, 2004.