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] Hudhayfah said: Whoever wakes up in a state of major impurity (janabah), he should fast that day. Marwan then said: I swear by Allah, O Abdur-Rahman, you must go to the Mothers of the Believers, Aisha and Umm Salamah, and ask them about that. Abu Bakr said: So Abdur-Rahman went and I went with him until we entered upon Aisha. Abdur-Rahman greeted her and said: O Mother of the Believers, we were with Marwan and it was mentioned to him that Abu Hurayrah said: Whoever wakes up in a state of major impurity (janabah), he should fast that day. Aisha said: It is not as Abu Hurayrah said. O Abdur-Rahman, do you desire something that the Messenger of Allah ﷺ did? Abdur-Rahman said: No, by Allah. Aisha said: Then I bear witness to the Messenger of Allah ﷺ that he would wake up in a state of major impurity (janabah) from sexual intercourse, other than a wet dream, and then he would fast that day. Aisha said: Afterwards we left until we entered upon Umm Salamah. Abdur-Rahman asked her about that, and she said the same thing as Aisha. Then we left until we came to Marwan, and Abdur-Rahman said to him: What did they say? Marwan said: I swear by you, O Abu Muhammad, that you must ride my mount at the door (of the Masjid), and go to Abu Hurayrah and inform him of that. So Abdur-Rahman rode, and I rode with him, until we came to Abu Hurayrah and Abdur-Rahman spoke to him for a while. Then he mentioned that to him, and Abu Hurayrah said: I have no knowledge of that. Only one informant told me  

هُرَيْرَةَ يَقُولُ مَنْ أَصْبَحَ جُنُبًا أَفْطَرَ ذَلِكَ الْيَوْمَ فَقَالَ مَرْوَانُ أَقْسَمْتُ عَلَيْكَ يَا عَبْدَ الرَّحْمَنِ لَتَذْهَبَنَّ إِلَى أُمِّ الْمُؤْمِنِينَ عَائِشَةَ وَأُمِّ سَلَمَةَ فَلَتَسْأَلَنَّهُمَا عَنْ ذَلِكَ قَالَ أَبُو بَكْرٍ 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 قَالَ مَنْ أَصْبَحَ جُنُبًا أَفْطَرَ ذَلِكَ الْيَوْمَ فَقَالَتْ عَائِشَةُ لَيْسَ كَمَا قَالَ أَبُو هُرَيْرَةَ يَا عَبْدَ الرَّحْمَنِ أَتَرْغَبُ عَمَّا كَانَ رَسُولُ اللهِ ﷺ يَفْعَلُهُ؟ فَقَالَ عَبْدُ الرَّحْمَنِ لَا وَاللهِ فَقَالَتْ عَائِشَةُ ؓ فَأَشْهَدُ عَلَى رَسُولِ اللهِ ﷺ إِنْ كَانَ لَيُصْبِحُ جُنُبًا مِنْ جِمَاعٍ غَيْرِ احْتِلَامٍ ثُمَّ يَصُومُ ذَلِكَ الْيَوْمَ قَالَ ثُمَّ خَرَجْنَا حَتَّى دَخَلْنَا عَلَى أُمِّ سَلَمَةَ فَسَأَلَهَا عَنْ ذَلِكَ فَقَالَتْ مِثْلَ مَا قَالَتْ عَائِشَةُ فَخَرَجْنَا حَتَّى جِئْنَا مَرْوَانَ فَقَالَ لَهُ عَبْدُ الرَّحْمَنِ مَا قَالَتَا؟ فَقَالَ مَرْوَانُ أَقْسَمْتُ عَلَيْكَ يَا أَبَا مُحَمَّدٍ لَتَرْكَبَنَّ دَابَّتِي بِالْبَابِ فَلَتَأْتِيَنَّ أَبَا هُرَيْرَةَ فَلَتُخْبِرَنَّهُ بِذَلِكَ فَرَكِبَ عَبْدُ الرَّحْمَنِ وَرَكِبْتُ مَعَهُ حَتَّى أَتَيْنَا أَبَا هُرَيْرَةَ فَتَحَدَّثَ مَعَهُ عَبْدُ الرَّحْمَنِ سَاعَةً ثُمَّ ذَكَرَ ذَلِكَ لَهُ فَقَالَ أَبُو هُرَيْرَةَ لَا عِلْمَ لِي بِذَلِكَ إِنَّمَا أَخْبَرَنِي مُخْبِرٌ  

[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