Why do the Shia perform seven Takbirs at the start of the prayer? Some of the Shia perform 7 Takbeers at the beginning, and the Ansar perform it always, are there Hadith for this tradition?
The Hadith from the Shia books are many for performing seven Takbirat, six being Mustahab/recommended and one being the obligatory Takbirat al-Ihram.
The Hadith are in Wasa’il al-Shia Chapter 7, English translations found here: http://www.tashayyu.org/hadiths/wasail/salat/takbirat-al-ihram/chapter-7 , below is one of the Hadith:
…Allah’s Messenger (pbuhap) was in salat[prayer] and to his side was al-Husayn b. `Ali. So Allah’s Messenger (pbuhap) did takbir and al-Husayn (pbuh) did not respond with takbir. Then Allah’s Messenger (pbuhap) did takbir and al-Husayn did not respond with takbir.
So Allah’s Messenger (pbuhap) did not cease from doing takbir and al-Husayn (pbuh) did not take up the takbir so he did not respond until he completed seven takbirat. So al-Husayn (pbuh) responded (with) the takbir in the seventh.
Then Abu `Abdillah (pbuh) said: So it turned into the Sunna.
Also from al-islam.org, it is referenced inside Takbirah al-Ihram:
3. Before the Takbirah al-Ihram in each of the five daily prayers, there are six mustahabb takbir and the seventh takbir constitutes that obligatory Takbirah al-Ihram…
As for the Ansar, it has been made clear that in this day and age, the Jurisprudence from Imam al-Mahdi (pbuh) states that all 7 Takbirat are now obligatory for the obligatory prayers (see Chara’i’ al-Islam and others).