Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) says: I used to recognize the conclusion of the prayer of the Messenger of Allah (peace and blessings be upon him) with the takbeer. Amr says: I said to (Abu Ma’bad): Have you narrated this hadith to me? He said: No, I have not narrated this to you.
Hadith Referenceالفتح الربانی / أبواب الأذ كار الواردة عقب الصلاة / 1883
Hadith Gradingمحدثین:صحیح
Hadith Takhrij«أخرجه البخاري: 841، ومسلم: 583 ، (انظر مسند أحمد ترقيم الرسالة: 1933 ترقیم بيت الأفكار الدولية: 1933»
Brief Explanation
Benefits: … These two hadiths are, in fact, one and the same hadith. It appears highly probable that the generality of the first hadith should be understood in the light of the specificity of the second hadith; that is, the issue established is that only "Allahu Akbar" should be pronounced aloud. As for some other hadiths in which it is established that the Prophet sallallahu alayhi wa sallam recited certain other supplications aloud, the intent behind those is to teach the people.
The meaning of Amr speaking to his teacher Abu Ma‘bad is that Abu Ma‘bad narrated the hadith to Amr, but after narrating it, he forgot. Therefore, when his student Amr asked him about it, he denied it. If a teacher narrates a hadith and then forgets it, then according to the most correct opinion, if the student is trustworthy, his narration will be considered authoritative. For details on this, see the discussion in the principles of hadith under "Man Haddatha wa Nasiya." (Further: Musnad Imam Ahmad, Muhaqqiq: 3/407) (Abdullah Rafiq)