The Prophet. said, "The biggest of the great sins are: To join others in worship with Allah, to be undutiful to one's parents, and to give a false witness." He repeated it thrice, or said, "....a false statement," and kept on repeating that warning till we wished he would stop saying it. (See Hadith No.7, Vol. 8)
Sahih al-Bukhari Book 88, Hadith 2
← Previous Hadith When the Verse: 'It is those who believe and confuse not their belief with wrong (i.e., worshipping others besides Allah): (6.82) was revealed, it became very hard on the companions of the Prophet (ﷺ) and they said, "Who among us has not confused his belief with wrong (oppression)?" On that, Allah's Apostle said, "This is not meant (by the Verse). Don't you listen to Luqman's statement: 'Verily! Joining others in worship with Allah is a great wrong indeed.' (31.13)

Sahih al-Bukhari Book 88, Hadith 1 Next Hadith → A bedouin came to the Prophet (ﷺ) and said, "O Allah's Messenger (ﷺ)! What are the biggest sins?: The Prophet (ﷺ) said, "To join others in worship with Allah." The bedouin said, "What is next?" The Prophet (ﷺ) said, "To be undutiful to one's parents." The bedouin said "What is next?" The Prophet (ﷺ) said "To take an oath 'Al-Ghamus." The bedouin said, "What is an oath 'Al-Ghamus'?" The Prophet (ﷺ) said, "The false oath through which one deprives a Muslim of his property (unjustly).

Sahih al-Bukhari Book 88, Hadith 3