Musnad Ahmad Hadith 5185

Hafs bin Asim says that once I was traveling with Ibn Umar (may Allah be pleased with them), he performed the Zuhr and Asr prayers as two rak'ahs each, then stood on his mat and saw some people offering voluntary prayers after the obligatory prayer. He asked, "What are these people doing?" I told him that they are offering voluntary prayers. He said, "If I were to offer voluntary prayers, would I not complete my obligatory prayer (why would I shorten it)?" I had the honor of accompanying the Prophet (peace be upon him) until his passing, and he never offered more than two rak'ahs, and likewise with Abu Bakr, Umar, and Uthman (may Allah be pleased with them).