Abdullah bin Masud reported,
When the verse "Those who believe and do not mix their faith with wrongdoing" [Anam 6:82] was revealed, it weighed heavily on the companions of the Messenger of Allah ﷺ, and they said, "Which of us does not wrong himself?"The Messenger of Allah ﷺ replied, "It is not as you suppose. It is as Luqman said to his son, 'And when Luqman said to his son, instructing him, My son, do not associate anything with Allah. Indeed, shirk is a great wrong'" [Luqman 31:13].
Azami 228
Sahihعن عبد الله بن مسعود قال
لما نزلت {الذين ءامنوا ولم يلبسوا إيمنهم بظلم} [سورة الأنعام: ٨٢] شق ذلك على أصحاب رسول الله ﷺ وقالوا: أينا لا يظلم نفسه؟ فقال رسول الله ﷺ: ليس هو كما تظنون، كما قال لقمان لابنه: {وإذ قال لقمن لابنه وهو يعظه يبنى لا تشرك بالله إن الشرك لظلم عظيم} [سورة لقمان: ١٣].
Bukhari narrated it in the chapters on the prophets (3360), and Muslim in the book of faith (124), both from the hadith of Amash, from Ibrahim, from Alqama, from Abdullah, who related the hadith.Shirk was called injustice because the essence of injustice is placing something in other than its proper place, and whoever commits shirk has set up a partner alongside Allah, which is among the greatest forms of injustice.