What is the meaning of the Hadeeeth indicating that people led in prayer must follow the Imaam in Salaah (prayer)? Would it be obligatory to follow the Imaam if he deliberately makes an extra obligatory action in Salaah?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said that "worshippers must follow the Imaam in the actions of Salah (prayer). If the Imaam says "Allaahu Akbar (Allaah is the Greatest)", those who follow must say the same. If the Imaam bows, those who follow must bow. If the Imaam says, "Sami‘a Allaahu liman Hamida (Allaah hears  the one who praises Him)", then those who follow must say, "Rabbana wa lakal hamd (Our Lord, to You is the praise)." Al-Bukhari and Muslim narrated this Hadeeth on the authority of Anas .

An-Nawawi, a Shaafi‘i scholar, said that the Hadeeth means that the worshippers must do the actions of Salah immediately after the Imaam. He said the Shaafi‘i school of Fiqh (jurisprudence) maintains that the Hadeeth refers to the actions of Salah, not the intention. In other words, a person is allowed to do an obligatory Salah while being led by an Imaam who is doing a voluntary Salah. However, this was not permitted by Maalik and Abu Haneefah . They said that the Hadeeth means that the worshipper must follow the Imaam in both the intention and the actions of Salah. This great Hadeeth has contains several Fiqh issues.

However, if the Imaam makes an extra Rak‘ah (unit of prayer) or an extra obligatory action of Salah, whether deliberately or not, a person must not follow him in doing so. If a person follows the Imaam in this extra action deliberately, while being aware that it is invalid, then his Salah will be invalid.

This was discussed before in Fatwas No. 1331 and 8330.

Allaah Knows best.