‘Amr ibn Maymun reported: Abdullah ibn Mas’ud, may Allah be pleased with him, said, “Verily, the majority of groups have left the united community. The united community is only the one that conforms to the obedience of Allah, even if you are by yourself.”
Source: Sharḥ Uṣūl I’tiqād Ahl al-Sunnah wal-Jamā’ah 160
عَنْ عَمْرِو بْنِ مَيْمُونٍ قال عبد الله بن مسعود رضي الله عنه إِنَّ جُمْهُورَ الْجَمَاعَةِ هِيَ الَّتِي تُفَارِقُ الْجَمَاعَةَ إِنَّمَا الْجَمَاعَةُ مَا وَافَقَ طَاعَةَ اللَّهِ وَإِنْ كُنْتَ وَحْدَكَ
160 شرح أصول اعتقاد أهل السنة والجماعة