In the Name of Allah, Most Gracious, Most Merciful.

All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.

Dear questioner, thanks for your question which reflects your keenness to follow the guidance of the Prophet (peace and blessings be upon him).

When performing acts of worship, a Muslim should be keen to follow the prophetic guidance on how these acts should be performed, so as not to fall into the trap of innovating groundless practices that negatively affect one's worship.  

Answering this question, Sheikh Muhammad SalehAl-Munajjid, a prominent Saudi lecturer and author, stated,

Reciting the Talbiyah (saying labbayka Allahumma labbayk) in unison and in one voice by the pilgrims performing `Umrah is one of the mistakes that many people commit today. Neither the Prophet (peace and blessings be upon him) nor his Companions were reported to have recited the Talbiyah in unison. So, each pilgrim should recite the Talbiyah alone, regardless of whether his or her Talbiyah will coincide with the Talbiyah of other pilgrims.

The Standing Committee for Ifta' and Scholarly Research was asked, "What is the ruling on the pilgrims who recite the Talbiyah together, where one recites it and the others repeat after him or her?"

The committee replied, "That is not permissible, because neither the Prophet (peace and blessings be upon him) nor his successors, the Rightly Guided Caliphs (may Allah be pleased with them), were reported to have done this; therefore, it is a bid`ah [innovation in religion]."

Sheikh Muhammad ibn Al-`Uthaymeen (may Allah have mercy on him) said,

Anas (may Allah be pleased with him) said, "We did Hajj with the Prophet (peace and blessings be upon him), and some of us were reciting Takbir [saying, 'Allah is Greatest'] and some of us were reciting Tahlil [saying, 'There is no god But Allah']" [Al-Bukhari and Muslim]. His words indicate that they did not recite the Talbiyah in unison. If they did recite the Talbiyah in unison, then they would all have been reciting Tahlil or Takbir [in unison], but some of them were reciting Takbir and some of them were reciting Tahlil. Each one of them was making remembrance of Allah according to his or her situation.

Excerpted from www.islam-qa.com