It is narrated with the previous chain that the Prophet Muhammad (peace be upon him) said: When the imam says the takbeer, you should also say the takbeer; when he bows (in ruku‘), you should also bow; when he prostrates, you should also prostrate; and when he prays sitting, then all of you should also pray sitting.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9652