Reciting the Talbiyah in unison, when the people doing ‘Umrah all start to recite the Talbiyah together and end it together in one voice is one of the mistakes that many people commit today. It is not known that the Prophet (peace and blessings of Allaah be upon him) or his companions recited the Talbiyah in unison. Rather each of them should recite the Talbiyah by himself, whether it coincides with the voices of other pilgrims who are reciting the Talbiyah or not. 

See the answer to question no. 33746. 

The Standing Committee for Issuing Fatwaa was asked: What is the ruling on pilgrims reciting the Talbiyah together, where one recites it and the others repeat after him? 

They replied: 

That is not permissible, because it was not narrated from the Prophet (peace and blessings of Allaah be upon him) or from his successors, the Rightly Guided Caliphs (may Allaah be pleased with them); rather it is an innovation (bid’ah). End quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: 

The words of Anas (may Allaah be pleased with him): “We did Hajj with the Prophet (peace and blessings of Allaah be upon him) and some of us were reciting takbeer (‘Allaahu akbar’) and some of us were reciting tahleel (‘La ilaaha ill-Allaah’)” (agreed upon) indicate that they did not recite the Talbiyah in unison. If they had been reciting the Talbiyah in unison then they would all have been reciting tahleel or takbeer, but some of them were reciting takbeer and some of them were reciting tahleel, but each of them was remembering his Lord according to his situation. End quote. 

Al-Sharh al-Mumti’ (7/111) 

And Allaah knows best.