Mishkat al-Masabih #150

Ibn Mas'ud reported God’s messenger as saying, “There was no prophet whom God raised up among his people before me who did not have from among his people apostles and companions who held to his sunna and followed what he commanded; then they were succeeded by people who said what they did not practise and did things they were not commanded to do. So he who strives against them with his hand is a believer, he who strives against them with his tongue is a believer, and he who strives against them with his heart is a believer. Beyond that there is not so much faith as a grain of mustard seed.” Muslim transmitted it.