Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) states that the Messenger of Allah (peace and blessings be upon him) said: "There was no prophet whom Allah, the Lord of Might, sent before me except that he had disciples and companions from his nation who took his Sunnah and followed it. Then after them came unworthy successors who said what they did not do and did what they were not commanded. So whoever strives against them with his hand is a believer, whoever strives against them with his tongue is a believer, and whoever strives against them with his heart is a believer; beyond that there is not even a mustard seed's weight of faith."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20178