Narrated by Abdullah bin Amr.

A bedouin came to the Prophet and said,
"O Allah's Apostle! What are the biggest sins?" The Prophet said, "To join others in worship with Allah." The bedouin said, "What is next?" The Prophet said, "To be undutiful to one's
parents." The bedouin said "What is next?"
The Prophet said "To take an oath 'Al-Ghamus." The
bedouin said, "What is an oath 'Al-Ghamus'?"
The Prophet said, "The false oath through which one
deprives a Muslim of his property (unjustly)."

Sahih Al Bukhari 9:55