(a) Imam al-Shafi'i (may Allah have mercy on him), after narrating from Imam Malik (may Allah have mercy on him) the hadith of Sayyiduna Umar (may Allah be pleased with him) on the tashahhud, said: This is the tashahhud that those of our jurists who preceded us in knowledge taught us when we were young. Then we heard it with its chain, and we also heard what differs from it. Our view was that Sayyiduna Umar (may Allah be pleased with him), on the minbar of the Messenger of Allah (peace and blessings be upon him) and in the midst of the Companions of the Messenger of Allah (peace and blessings be upon him), would teach the people only what the Prophet (peace and blessings be upon him) had taught them. Then, when a hadith reached us from our companions that we hold to be established from the Prophet (peace and blessings be upon him), we adopted it, and it was more fitting for us. Then he mentioned the hadith of Ibn Abbas (may Allah be pleased with them both). A man who debated Imam al-Shafi'i (may Allah have mercy on him) on this said: We see that the narration of it from the Prophet (peace and blessings be upon him) has itself differed: Ibn Mas'ud (may Allah be pleased with him) narrated something different from this, and Abu Musa and Jabir (may Allah be pleased with them both) also narrated it, and their narrations differ somewhat from one another in wording.
Then Sayyiduna Umar (may Allah be pleased with him) taught something different from this. All of this is, in essence, a difference of wording. Likewise is the tashahhud of Sayyidah Aishah (may Allah be pleased with her) and Ibn Umar (may Allah be pleased with them both); that is, some of them added something over others.
(b) Imam al-Shafi'i (may Allah have mercy on him) says: Every speech by which the glorification of Allah is intended was taught to them by the Messenger of Allah (peace and blessings be upon him); one of them memorised it with one wording and another memorised it with a different wording. The words differ, but the meaning is one.
Perhaps the Messenger of Allah (peace and blessings be upon him) permitted each of them to recite it as he had memorised it, as long as the meaning does not change and its ruling does not change. And he cited as evidence for this the hadith of the (seven) modes (huruf) of the Qur'an.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2844