One may begin to fast even in a state of ritual impurity

At times Messenger of Allah ﷺ used to get up in the morning in the state of Janaba after having sexual relations with his wives. He would then take a bath and fast.  

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ يُدْرِكُهُ الْفَجْرُ وَهُوَ جُنُبٌ مِنْ أَهْلِهِ ثُمَّ يَغْتَسِلُ وَيَصُومُ  

Yahya related to me from Malik from Sumayy, the mawla of Abu Bakr ibn Abd ar-Rahman ibn al-Harith ibn Hisham that he heard Abu Bakr ibn Abd ar-Rahman ibn al-Harith ibn Hisham say, "My father and I were with Marwan ibn al Hakam at the time when he was amir of Madina, and someone mentioned to him that Abu Hurayra used to say, 'If someone begins the morning junub, he has broken the fast for that day.' Marwan said, 'I swear to you, Abdar-Rahman, you must go to the two umm al muminin, A'isha and Umm Salama, and ask them about it.' ''Abd ar-Rahman went to visit A'isha and I accompanied him. He greeted her and then said, 'Umm al-muminin, we were with Marwan ibn al Hakam and someone mentioned to him that Abu Hurayra used to say that if some one had begun the morning junub, he had broken the fast for that day.' A'isha said, 'It is not as Abu Hurayra says Abd ar-Rahman. Do you dislike what the Messenger of Allah ﷺ used to do?', and Abd ar-Rahman said, 'No, by Allah.' A'isha said, 'I bear witness that the Messenger of Allah ﷺ used to get up in the morning junub from intercourse, not a dream, and would then fast for that day.' " He continued, "Then we went and visited Umm Salama, and Abd ar- Rahman asked her about the same matter and she said the same as A'isha had said. Then we went off until we came to Marwan ibn al-Hakam Abd ar-Rahman told him what they had both said and Marwan said, 'I swear to you, Abu Muhammad, you must use the mount which is at the door, and go to Abu Hurayra, who is on his land at al Aqiq, and tell him this.' So Abd ar-Rahman rode off, and I went with him, until we came to Abu Hurayra. Abd ar-Rahman talked with him for a while, and then mentioned the matter to him, and Abu Hurayra said, 'I don't know anything about it. I was just told that by someone.'"  

يَقُولُ كُنْتُ أَنَا وَأَبِي عِنْدَ مَرْوَانَ بْنِ الْحَكَمِ وَهُوَ أَمِيرُ الْمَدِينَةِ فَذُكِرَ لَهُ أَنَّ أَبَا هُرَيْرَةَ يَقُولُ مَنْ أَصْبَحَ جُنُبًا أَفْطَرَ ذَلِكَ الْيَوْمَ فَقَالَ مَرْوَانُ أَقْسَمْتُ عَلَيْكَ يَا عَبْدَ الرَّحْمَنِ لَتَذْهَبَنَّ إِلَى أُمَّىِ الْمُؤْمِنِينَ عَائِشَةَ وَأُمِّ سَلَمَةَ فَلَتَسْأَلَنَّهُمَا عَنْ ذَلِكَ فَذَهَبَ عَبْدُ الرَّحْمَنِ وَذَهَبْتُ مَعَهُ حَتَّى دَخَلْنَا عَلَى عَائِشَةَ فَسَلَّمَ عَلَيْهَا ثُمَّ قَالَ يَا أُمَّ الْمُؤْمِنِينَ إِنَّا كُنَّا عِنْدَ مَرْوَانَ بْنِ الْحَكَمِ فَذُكِرَ لَهُ أَنَّ أَبَا هُرَيْرَةَ يَقُولُ مَنْ أَصْبَحَ جُنُبًا أَفْطَرَ ذَلِكَ الْيَوْمَ قَالَتْ عَائِشَةُ لَيْسَ كَمَا قَالَ أَبُو هُرَيْرَةَ يَا عَبْدَ الرَّحْمَنِ أَتَرْغَبُ عَمَّا كَانَ رَسُولُ اللَّهِ ﷺ يَصْنَعُ فَقَالَ عَبْدُ الرَّحْمَنِ لاَ وَاللَّهِ قَالَتْ عَائِشَةُ فَأَشْهَدُ عَلَى رَسُولِ اللَّهِ ﷺ أَنَّهُ كَانَ يُصْبِحُ جُنُبًا مِنْ جِمَاعٍ غَيْرِ احْتِلاَمٍ ثُمَّ يَصُومُ ذَلِكَ الْيَوْمَ قَالَ ثُمَّ خَرَجْنَا حَتَّى دَخَلْنَا عَلَى أُمِّ سَلَمَةَ فَسَأَلَهَا عَنْ ذَلِكَ فَقَالَتْ مِثْلَ مَا قَالَتْ عَائِشَةُ قَالَ فَخَرَجْنَا حَتَّى جِئْنَا مَرْوَانَ بْنَ الْحَكَمِ فَذَكَرَ لَهُ عَبْدُ الرَّحْمَنِ مَا قَالَتَا فَقَالَ مَرْوَانُ أَقْسَمْتُ عَلَيْكَ يَا أَبَا مُحَمَّدٍ لَتَرْكَبَنَّ دَابَّتِي فَإِنَّهَا بِالْبَابِ فَلْتَذْهَبَنَّ إِلَى أَبِي هُرَيْرَةَ فَإِنَّهُ بِأَرْضِهِ بِالْعَقِيقِ فَلْتُخْبِرَنَّهُ ذَلِكَ فَرَكِبَ عَبْدُ الرَّحْمَنِ وَرَكِبْتُ مَعَهُ حَتَّى أَتَيْنَا أَبَا هُرَيْرَةَ فَتَحَدَّثَ مَعَهُ عَبْدُ الرَّحْمَنِ سَاعَةً ثُمَّ ذَكَرَ لَهُ ذَلِكَ فَقَالَ لَهُ أَبُو هُرَيْرَةَ لاَ عِلْمَ لِي بِذَاكَ إِنَّمَا أَخْبَرَنِيهِ مُخْبِرٌ  

[Machine] Abu Hurairah said: "Whoever wakes up in a state of janabah (ritual impurity after intercourse) should break his fast that day." Marwan said: "I swear by Allah, O Abdul-Rahman, you must go to Ummul Mu'mineen Aishah and Umm Salamah and ask them about this." So Abdul-Rahman went and I went with him until we entered upon Aishah. Abdul-Rahman greeted her and said: "O Mother of the Believers, we were with Marwan and he mentioned to him what Abu Hurairah said: 'Whoever wakes up in a state of janabah should break his fast that day.' Aishah said: 'I bear witness to the Messenger of Allah ﷺ that he used to wake up in a state of janabah due to intercourse, not due to a wet dream, and he would fast that day.' Then we left and went to Umm Salamah and asked her. She said the same as Aishah. Then we went to Marwan and Abdul-Rahman told him what they had said. Marwan said: 'I swear by Allah, O Abu Muhammad, you must ride my she-camel, for it is at the gate. Go to Abu Hurairah, for he is in his land in Al-'Aqiq, and inform him of that.' Abu Bakr said: Abdul-Rahman rode (the she-camel) and I rode with him until we reached Abu Hurairah. Abdul-Rahman spoke with him for a while, then he mentioned it to him. Abu Hurairah said: 'I have no knowledge about it, I was only told this by a narrator.' Abu Abdullah (Imam Ahmad) said: Abu Qilabah narrated from Abdul-Rahman bin Al-Harith that he informed Abu Hurairah about the statement of Aishah and Umm Salamah. Abu Hurairah said: 'That is what I thought, but it was not allowed for anyone to do it except him.'"  

أَنَّ أَبَا هُرَيْرَةَ يَقُولُ مَنْ أَصْبَحَ جُنُبًا أَفْطَرَ ذَلِكَ الْيَوْمَ قَالَ مَرْوَانُ أَقْسَمْتُ عَلَيْكَ يَا عَبْدَ الرَّحْمَنِ لَتَذْهَبَنَّ إِلَى أُمَّيِ الْمُؤْمِنِينَ عَائِشَةَ وَأَمِّ سَلَمَةَ فَلْتَسْأَلَنَّهُمَا عَنْ ذَلِكَ فَذَهَبَ عَبْدُ الرَّحْمَنِ وَذَهَبْتُ مَعَهُ حَتَّى دَخَلْنَا عَلَى عَائِشَةَ فَسَلَّمَ عَلَيْهَا عَبْدُ الرَّحْمَنِ فَقَالَ يَا أُمَّ الْمُؤْمِنِينَ إِنَّا كُنَّا عِنْدَ مَرْوَانَ فَذُكِرَ لَهُ أَنَّ أَبَا هُرَيْرَةَ يَقُولُ مَنْ أَصْبَحَ جُنُبًا أَفْطَرَ ذَلِكَ الْيَوْمَ قَالَتْ عَائِشَةُ «أَشْهَدُ عَلَى رَسُولِ اللهِ ﷺ أَنَّهُ كَانَ يُصْبِحُ جُنُبًا مِنْ جِمَاعٍ غَيْرِ احْتِلَامٍ ثُمَّ يَصُومُ ذَلِكَ الْيَوْمَ» ثُمَّ خَرَجْنَا فَدَخَلْنَا عَلَى أُمِّ سَلَمَةَ فَسَأَلَهَا فَقَالَتْ كَمَا قَالَتْ عَائِشَةُ فَخَرَجْنَا حَتَّى جِئْنَا مَرْوَانَ فَذَكَرَ لَهُ عَبْدُ الرَّحْمَنِ مَا قَالَتَا قَالَ مَرْوَانُ أَقْسَمْتُ عَلَيْكَ يَا أَبَا مُحَمَّدٍ لَتَرْكَبَنَّ دَابَّتِي فَإِنَّهَا بِالْبَابِ فَلْتَذْهَبَنَّ إِلَى أَبِي هُرَيْرَةَ فَإِنَّهُ بِأَرْضِهِ بِالْعَقِيقِ فَلَتُخْبِرَنَّهُ ذَلِكَ قَالَ أَبُو بَكْرٍ فَرَكِبَ عَبْدُ الرَّحْمَنِ وَرَكِبْتُ مَعَهُ حَتَّى أَتَيْنَا أَبَا هُرَيْرَةَ فَتَحَدَّثَ عَبْدُ الرَّحْمَنِ مَعَهُ سَاعَةً ثُمَّ ذَكَرَ لَهُ فَقَالَ أَبُو هُرَيْرَةَ لَا عِلْمَ لِي إِنَّمَا أَخْبَرَنِيهِ مُخْبِرٌ قَالَ أَبُو عَبْدِ الرَّحْمَنِ رَوَاهُ أَبُو قِلَابَةَ عَنْ عَبْدِ الرَّحْمَنِ بْنِ الْحَارِثِ أَنَّهُ أَخْبَرَ أَبَا هُرَيْرَةَ بِقَوْلِ عَائِشَةَ وَأَمِّ سَلَمَةَ فَقَالَ هَكَذَا كُنْتُ أَحْسِبُ وَلَمْ يُحِلْهُ عَلَى أَحَدٍ  

[Machine] "Abdullah al-Hafiz informed Abu Abdullah al-Shaybani, who narrated from Ibrahim ibn Abi Talib, who narrated from Muhammad ibn Rafi', who narrated from Abdul Razzaq, who narrated from Ibn Jurayj, who reported from Abdul Malik ibn Abi Bakr ibn Abdul Rahman, who narrated from his father Abu Bakr, meaning his father, who said: I heard Abu Huraira saying about his stories, 'Whoever experiences sexual emission before dawn should not fast.' Then I mentioned this to Abdul Rahman ibn Al-Harith, his father, and he denied it. So Abdul Rahman went and I went with him until we entered upon Aisha and Umm Salama. Abdul Rahman asked them about this matter, and they both said, 'The Messenger of Allah ﷺ would wake up in a state of junub (ritual impurity after sexual intercourse) sometimes without dreaming, and then he would fast.' So we left until we reached Marwan, and Abdul Rahman mentioned this to him. Marwan said, 'I wanted to scold you except that you went to Abu Huraira and mentioned to him what he said.' So we went to Abu Huraira, and Abu Bakr was present throughout all of this. Abdul Rahman mentioned this to him, and Abu Huraira said, 'I heard this from Fadl ibn Abbas, and I did not hear it from the Prophet ﷺ .' Then Abu Huraira abandoned what he used to say about this matter. I said to Abdul Malik, 'Did they say this about Ramadan?' He said, 'Yes, he would sometimes wake up in a state of junub without dreaming, and then he would fast.'"  

عَبْدِ اللهِ الْحَافِظُ أنبأ أَبُو عَبْدِ اللهِ الشَّيْبَانِيُّ ثنا إِبْرَاهِيمُ بْنُ أَبِي طَالِبٍ ثنا مُحَمَّدُ بْنُ رَافِعٍ ثنا عَبْدُ الرَّزَّاقِ أنبأ ابْنُ جُرَيْجٍ أَخْبَرَنِي عَبْدُ الْمَلِكِ بْنُ أَبِي بَكْرِ بْنِ عَبْدِ الرَّحْمَنِ عَنْ أَبِي بَكْرٍ يَعْنِي أَبَاهُ قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ يَقُولُ فِي قَصَصِهِ مَنْ أَدْرَكَهُ الْفَجْرُ جُنُبًا فَلَا يَصُمْ قَالَ فَذَكَرْتُ ذَلِكَ لِعَبْدِ الرَّحْمَنِ بْنِ الْحَارِثِ لِأَبِيهِ فَأَنْكَرَ ذَلِكَ فَانْطَلَقَ عَبْدُ الرَّحْمَنِ وَانْطَلَقْتُ مَعَهُ حَتَّى دَخَلْنَا عَلَى عَائِشَةَ وَأُمِّ سَلَمَةَ ؓ فَسَأَلَهُمَا عَبْدُ الرَّحْمَنِ عَنْ ذَلِكَ قَالَ فَكِلْتَاهُمَا قَالَتْ كَانَ رَسُولُ اللهِ ﷺ يُصْبِحُ جُنُبًا مِنْ غَيْرِ حُلْمٍ ثُمَّ يَصُومُ قَالَ فَانْطَلَقْنَا حَتَّى دَخَلْنَا عَلَى مَرْوَانَ فَذَكَرَ ذَلِكَ لَهُ عَبْدُ الرَّحْمَنِ فَقَالَ مَرْوَانُ عَزَمْتُ عَلَيْكَ إِلَّا مَا ذَهَبْتَ إِلَى أَبِي هُرَيْرَةَ وَرَدَدْتَ عَلَيْهِ مَا يَقُولُ قَالَ فَجِئْنَا أَبَا هُرَيْرَةَ وَأَبُو بَكْرٍ حَاضِرٌ ذَلِكَ كُلَّهُ قَالَ فَذَكَرَ ذَلِكَ لَهُ عَبْدُ الرَّحْمَنِ فَقَالَ أَبُو هُرَيْرَةَ سَمِعْتُ ذَلِكَ مِنَ الْفَضْلِ بْنِ عَبَّاسٍ وَلَمْ أَسْمَعْهُ مِنَ النَّبِيِّ ﷺ قَالَ فَرَجَعَ أَبُو هُرَيْرَةَ عَمَّا كَانَ يَقُولُ فِي ذَلِكَ قُلْتُ لِعَبْدِ الْمَلِكِ قَالَتَا فِي رَمَضَانَ؟ قَالَ كَذَلِكَ يُصْبِحُ جُنُبًا مِنْ غَيْرِ حُلْمٍ ثُمَّ يَصُومُ