Imam al-Shafi'i (may Allah have mercy on him) said: "My view is that regarding the report Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) narrated of the tahlil of the Prophet (peace and blessings be upon him) (that is, saying La ilaha illallah), and the report Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrated of his (peace and blessings be upon him) takbir (that is, saying Allahu akbar), exactly as these reports have reached us,
I think that he (peace and blessings be upon him) surely recited these words somewhat aloud only so that the people might learn (this remembrance) from him."
It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that he said: I used to recognise the end of the prayer of the Messenger of Allah (peace and blessings be upon him) by the takbir. Amr bin Dinar (may Allah have mercy on him) says: I mentioned this hadith before Abu Ma'bad, and he said: I did not narrate this hadith to you. Amr says: He did narrate this hadith to me, and he was the most truthful of the freed slaves of Ibn Abbas (may Allah be pleased with them both).
(b) Imam al-Shafi'i (may Allah have mercy on him) says: He must have forgotten it after narrating it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3013