Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that indeed the Imam is a trustee or a leader; if he leads the prayer sitting, then you should also pray sitting, and if he leads the prayer standing, then you should also pray standing.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب قيام المأمومين خلف الإمام وما فيه من السنن / 1613