He traced it back to the Prophet ﷺ, who said, "Were it not that I would impose hardship on my community, I would have commanded them to delay the night prayer and to use the tooth-stick with every prayer."
حدثنا سفيان عن أبي الزناد عن الأعرج
يبلغ به النبي ﷺ لولا أن أشق على أمتي لأمرتهم بتأخير العشاء والسواك مع الصلاة
Its chain of transmission is sahih. It appears clear from the manuscripts of the Musnad that this hadith and the one following it are treated as a single hadith, and that is why we numbered them with one number in our edition. But in reality they are two hadiths sharing one chain of transmission, and the second of them has another chain mentioned right after it. The first is about delaying the night prayer and about using the tooth-stick, and the second is about a woman fasting with her husband's permission. We have separated the two here, and we have given the second the same number repeated, marking it with the letter M beside it. The first of the two has already appeared with this chain, Sufyan from Abu Zinad from Araj, number 7335, and we noted it there.7338M, A woman must not fast, while her husband is present, on a day other than in Ramadan, except with his permission. This hadith was read aloud to him, I heard Abu Zinad, from Musa bin Abu Uthman, from his father, from Abu Hurairah, from the Prophet ﷺ.Sahih. (7338M) Both of its chains are sahih. Imam Ahmad reported it from Sufyan, from Abu Zinad, from Araj, from Abu Hurairah, with the same chain as the hadith before it. He then established that it was read aloud to Sufyan bin Uyayna, from Abu Zinad, from Musa bin Abu Uthman, from his father, from Abu Hurairah. His narration with both chains is established as coming from Sufyan bin Uyayna from Abu Zinad, and also from Sufyan Thawri from Abu Zinad, as we will mention in the source-tracing.Musa bin Abu Uthman, in the second chain, is trustworthy. Ibn Hibban mentioned him among the trustworthy narrators, and Bukhari gave his biography in al-Kabir 4/1/290, distinguishing him from another Musa bin Abu Uthman who narrates from Abu Yahya from Abu Hurairah. This latter one was narrated from by Thawri and Shuba, while the Tabban was narrated from by Abu Zinad, and Thawri also narrated from Abu Zinad from him. Ibn Abu Hatim likewise distinguished between the two in al-Jarh wal-Tadil 4/1/153, following Bukhari. Mizzi, however, treated them as one and the same in al-Tahdhib, and the Hafiz mentioned the distinction between them citing Ibn Abu Hatim alone. But Ibn Abu Hatim did nothing more than follow Bukhari, and he was correct in doing so.Al-Tabban, with a fatha on the letter ta and a doubled ba, is a designation referring to selling straw. His father was Abu Uthman al-Tabban, a freedman of Mughira bin Shuba, trustworthy. Ibn Hibban mentioned him among the trustworthy narrators, Tirmidhi graded one of his hadiths hasan, and Bukhari reported this hadith from him with a suspended chain in his Sahih, as will be mentioned.This hadith was reported by Darimi 2:12, Tirmidhi 2:66, and Ibn Majah 1761, all of them through the route of Sufyan bin Uyayna, from Abu Zinad, from Araj, from Abu Hurairah, the same as the first chain, meaning the chain of 7338. Bukhari reported it, 9:259-260, within a lengthy hadith, through the route of Shuayb, from Abu Zinad, from Araj, from Abu Hurairah.As for the second chain mentioned right after this hadith, which was read aloud to Sufyan bin Uyayna, it is also established. Bukhari referred to it, 9:261, following his narration within the lengthy hadith we mentioned, saying, Abu Zinad also reported it, from Musa, from his father, from Abu Hurairah, regarding fasting. Tirmidhi likewise referred to it, following his earlier narration, saying, this hadith has also been narrated from Abu Zinad, from Musa bin Abu Uthman, from his father, from Abu Hurairah, from the Prophet ﷺ. Sufyan Thawri likewise reported it from Abu Zinad from Musa, from his father. It will come by the route of Thawri at 9732, 9987, 10171, 10501, and Darimi also reported it 1:12 through the route of Thawri, likewise.The Hafiz said, explaining Bukhari's reference to the narration of Abu Zinad from Musa, he is indicating that the narration of Shuayb from Abu Zinad from Araj included three rulings, and that Abu Zinad has, for one of the three, namely the fasting of the woman, another chain. The Musa mentioned is Ibn Abu Uthman, and his father Abu Uthman is called al-Tabban, with a ta then a doubled ba, and his name is Saad, though some say Imran. He was a freedman of Mughira bin Shuba, and he appears in Bukhari only in this one place. His hadith mentioned here was also reported in full by Ahmad, Nasai, Darimi, and Hakim, through the route of Thawri, from Abu Zinad, from Musa bin Abu Uthman, with the story of fasting. Darimi as well. Ibn Khuzayma, Abu Awana, and Ibn Hibban reported it through the route of Sufyan bin Uyayna, from Abu Zinad, from Araj, with the same content. Abu Awana said, in the narration of Ali bin Madini, Sufyan later related it to us from Abu Zinad, from Musa bin Abu Uthman, so I checked with him about it, and he held firm on Musa and retracted the mention of Araj. We have also transmitted it with a higher chain, in the section of Ismail bin Nujayd, through the narration of Mughira bin Abdul-Rahman, from Abu Zinad.This is a valuable piece of source-tracing by the Hafiz, as is his habit, may Allah have mercy on him. We have indicated the places of his narration in the Musnad and in Sunan Darimi. I did not find his narration in Hakim or in Nasai, and the Hafiz, may Allah have mercy on him, overlooked pointing out Ahmad's two narrations here in the Musnad, from Sufyan bin Uyayna, with both versions.His saying, while her husband is present, means, while he is present. Al-shahid and al-shahid both mean, the one who is present.