Are we obliged to reduce prayer when on a journey?

I have an issue. I thought so long, we could pray in full prayer length, when we have just travelled. Yesterday, Our Imam came from another region. In the prayer of Al-asr and Isha (I did not pray with him Magrib). He just prayed 2 rakats instead of 4. After the 2 rakats, we stand up and pray the last 2 rakats alone. I asked the question to another one about that situation and he told me, our Imam came from a long journey, so He prays 2 Rakats. My question is: Is it an obligation to pray 2 rakats or we can pray in full length?