I heard that we have to make expiation if we passed in front of someone who is praying. Is this true? May Allaah The Almighty Reward you the best.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The Prophet, sallallaahu ‘alayhi wa sallam, prohibited passing in front of one who is praying, saying: “If the one who walks in front of another who is praying knew (the sin) that is upon him (for doing so), he would prefer standing for forty rather than passing in front of him.” [Al-Bukhari and Muslim]

There is no specific expiation for this. However, one who did so deliberately must repent and seek forgiveness from the sin he committed.