The Messenger of Allah ﷺ led us in one of the afternoon prayers, praying two units, then gave the closing salutation. He then mentioned the hadith, "so let him respond."
قال أبي وحدثنا قبله قال حدثنا هشام وابن عون عن محمد
صلى بنا رسول الله ﷺ إحدى صلاتي العشي ركعتين ثم سلم فذكر الحديث فليجب
Its chain of transmission is sahih. This hadith belongs to the musnad of Abu Hurairah, but establishing it here alongside the preceding chain requires investigation. It appears that Hammad bin Usama narrated to Ahmad the hadith of Ibn Umar about responding to an invitation, no. 4949, from Ubaydullah, from Nafi, from Ibn Umar, on one occasion, and that he narrated it to him with the same chain on another occasion, but did not state its wording. Instead he said, "This hadith, and this description," which is chain no. 4950. This occurred right after he had narrated to him the hadith of Abu Hurairah about one of the two afternoon prayers, which is the story of Dhul-Yadayn regarding the prostration of forgetfulness, along with his hadith about responding to an invitation. He combined for him the two hadiths of Abu Hurairah into a single hadith with a single chain, from Hisham bin Hassan and Ibn Awn, from Muhammad bin Sirin, from Abu Hurairah. Both hadiths were narrated by Abu Hurairah, as we will mention.When Ahmad heard chain no. 4950 from his teacher Hammad bin Usama, from Ubaydullah, from Nafi, from Ibn Umar, right after the two hadiths of Abu Hurairah which had been combined into one, and heard him say regarding the chain of Ibn Umar's hadith, "This hadith, and this description," he had doubts about this latter hearing. I mean, he doubted the correctness of narrating the entire hadith, in both its parts, from Ibn Umar, the story of Dhul-Yadayn and the response to an invitation together. So he recorded chain no. 4950 right after 4949, even though they are a single chain, then explained how his teacher had narrated the chain to him the second time. This is a precise measure of caution on the part of the Imam, for the story of Dhul-Yadayn is well preserved and known from the hadith of Abu Hurairah, narrated by the two Shaykhs (Bukhari and Muslim) and others, as in al-Muntaqa 1326, and it will come in his Musnad through many chains, including 7370, 7563, 7807, 9458, 9927. Indeed it also appears at 7200, from the hadith of Muhammad bin Abu Adi, from Ibn Awn, from Muhammad bin Sirin, from Abu Hurairah, who said, "The Messenger of Allah ﷺ prayed one of the two afternoon prayers," he said, "Abu Hurairah mentioned it while Muhammad forgot it," "so he prayed two units, then said the closing salutation, and went to a piece of wood laid across the masjid, and placed his hand on it as if he were angry. The hasty ones left through the doors of the masjid, saying, 'The prayer has been shortened!' " He said, "Among the people were Abu Bakr and Umar, but they were too in awe of him to speak to him. Among the people was a man with long arms, called Dhul-Yadayn, who said, 'O Messenger of Allah, did you forget, or was the prayer shortened?' " He said, "I did not forget, nor was it shortened," and so on.I did not find it in the Musnad from the hadith of Hisham bin Hassan, from Ibn Sirin, so its addition can be drawn from this passage. The hadith about responding to an invitation is also established from Abu Hurairah, in the Sahihayn and elsewhere, as in al-Muntaqa 3577, 3578, and it will come in his musnad repeatedly. Indeed it will come with wording similar to that of Ibn Umar, through the narration of Hisham bin Hassan, from Ibn Sirin, from Abu Hurairah, at 7735 and 10593, and through the narration of Ayub, from Ibn Sirin, from Abu Hurairah, at 10354. This is the text of chain 10593, "Yazid narrated to us, Hisham informed us, from Muhammad, from Abu Hurairah, from the Prophet ﷺ, who said, 'When one of you is invited, let him accept. If he is fasting, let him pray, and if he is not fasting, let him eat.' " I did not find it in the Musnad through the narration of Ibn Awn, from Ibn Sirin, so its addition can also be drawn from this passage.Furthermore, I did not find the story of Dhul-Yadayn narrated from the hadith of Ibn Umar in the Musnad except in this passage, through this indication from Imam Ahmad, from his teacher Hammad bin Usama. Nor did I find it in any of the hadith compilations except through the narration of Hammad bin Usama. Abu Dawud narrated it, 1:389, from Ahmad bin Muhammad bin Thabit and Abu Kurayb Muhammad bin Ala, both from Abu Usama, who is Hammad bin Usama, with chain 4950, and he did what Ahmad bin Hanbal did here, not giving its wording in full, but saying, from Ibn Umar, "The Messenger of Allah ﷺ led us in prayer and gave the closing salutation after two units," then he related something similar to the hadith of Ibn Sirin from Abu Hurairah, saying, "Then he gave the closing salutation, then he performed the two prostrations of forgetfulness." Before that, he had narrated the hadith of Abu Hurairah through chains via Ibn Sirin, 1:385–388, then through other chains not via Ibn Sirin, then he narrated the hadith of Hammad bin Usama, and referred it to something similar to the hadith of Ibn Sirin, as we mentioned.Ibn Majah narrated it, 1:189, from Ali bin Muhammad, Abu Kurayb, and Ahmad bin Sinan, all three from Abu Usama Hammad bin Usama, with chain 4950. Bayhaqi narrated it, 2:359, through the route of Abu Kurayb, from Abu Usama, likewise, but both of them gave the wording of the hadith more briefly than the narration of Ibn Sirin from Abu Hurairah. Bayhaqi then said, "Abu Usama Hammad bin Usama alone narrated it."This, then, is the point of caution exercised by the Imam, may Allah have mercy on him. He found that his teacher Abu Usama was the sole narrator of this account, and that he had narrated it to them right after the two hadiths of Abu Hurairah about the story of Dhul-Yadayn and responding to an invitation, which he had narrated with a single chain, and then he narrated to them, from Ubaydullah, from Nafi, from Ibn Umar, from the Prophet ﷺ, "This hadith, and this description." He found that the hadith about responding to an invitation is established from Ibn Umar through a different chain, while the story of Dhul-Yadayn was not narrated from Ibn Umar through any other chain. So he feared that his teacher Abu Usama might have meant, by his statement "this hadith, and this description," only the latter part of the preceding hadith, namely responding to an invitation, and not the whole hadith in both its parts, the story of Dhul-Yadayn together with responding to an invitation. It appears that he had not heard from Abu Usama the story of Dhul-Yadayn alone from the hadith of Ibn Umar, so he exercised caution and related exactly what he had heard.As for the others, Ahmad bin Muhammad bin Thabit, Abu Kurayb, Ali bin Muhammad, and Ahmad bin Sinan, it appears that they heard from Abu Usama the hadith of Ibn Umar about the story of Dhul-Yadayn separately from the hadith of Abu Hurairah, and some of them heard it both separately and combined, so some of them recorded its wording and full context. Had Ahmad heard what these others heard, he would not have exercised this degree of caution, for Hammad bin Usama was trustworthy, a hafiz of precise memory, whose solitary narrations are accepted as evidence. Ahmad said, "Abu Usama is more reliable than a hundred like Abu Asim, he had a sound book and precise command of hadith, sharp-minded and truthful." He also said, "He was reliable, how reliable he was, he hardly ever erred."