It is false to say that enduring oppression is a sin. This notion has no basis in the Quran and Hadith. This is why it is false. Where is it written in the Quran or Hadith that suffering oppression is unlawful? The Prophet suffered oppression in Makkah for 13 years. Did he commit a sin, God forbid? A person called Yasir had accepted Islam in Makkah. He belonged to the weaker section. People persecuted Yasir and his family. Once the Prophet saw people beating up Yasir. Yasir said: "O Prophet, people are hitting me." The Prophet replied: "O people of Yasir, be patient. Your abode is Paradise." (Shuab al-Iman, al-Bayhaqi 1515)
This is an example of enduring oppression. Nowhere does the Quran say that bearing with oppression is a sin. Such notions are false. If a notion has not been mentioned in the Quran or Hadith, it will be considered false. People generally listen to what others say without asking for reference.  Muslims do not have the attitude to seek reference from the Quran and Hadith for a statement. According to a hadith: "It is sufficient for a person to be a liar if he repeats everything he hears." (Sahih Muslim v. 1, p. 10)

At first you must ask the speaker for a reference for his claims. You should ask him where the Quran says that enduring oppression is a sin? If a person simply repeats everything that he has heard, then he is a liar.  In Islam, investigating a matter is obligatory. The Quran says: يَا أَيُّهَا الَّذِينَ آمَنُوا إِنْ جَاءَكُمْ فَاسِقٌ بِنَبَإٍ فَتَبَيَّنُوا "Believers, if an evil-doer brings you news, ascertain the correctness of the report fully." (49:6) You must not hear something and simply go about repeating it. In God's eyes such a person is a liar. This is indeed very serious. Muslims tend to simply repeat anything that they hear.