Tajdeed

Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad ibn ‘Isa from al-Husayn ibn Sa‘id from ‘Uthman ibn ‘Isa from Mansur ibn Hazim from Bakr ibn Habib who has said the following: “I once asked abu Ja‘far (a.s), about Tashahhud (the testimony of belief). He (the Imam) said, ‘Had it been as they say that it was obligatory people would have been destroyed. People only said what was easy of what they knew. If you praise Allah it is sufficient.’”
Reference: Al-Kafi, Volume 3, Book 4, Chapter 30, Hadith 1:

I am relating this report attributed to the great Imam of Ahlulbayt, Imam Abu Ja’far Muhammad al-Baqir (a.s.) because many lay people are confused regarding the subtle, yet irrelevant differences within some legal issues.

The fact of the matter is that there was a rukhsa (permission) given by Allah and His noble Messenger (s) regarding some of the issues. The ultimate priority is to uphold the Rope of Allah and hold fastly to it for guidance. This means that whenever a definitive aspect of Deen is established via clear, explicit evidence, then we cannot quarrel or disunite over it but instead we are to firmly fulfil the obligation or refrain from the filth of haram.

However, whenever there is an ambiguous matter, or something that was intentionally left with flexibility, then we cannot disunite over such a thing. The example of different Tashhud is one of those things. After all, how can the Muslim Ummah differ so much on a very practical ritual that is practiced five times a day, if it was supposed to be rigid and in a univocal form. Thus, the differences are an indication of the leniency in this particular matter, so embrace it!