Sahih Muslim, Chapter: Faith (Kitab Al Iman)

that the Messenger of Allah (may peace and blessings be upon him) observed: Never a Prophet had been sent before me by Allah towards his nation who had not among his people (his) disciples and companions who followed his ways and obeyed his command. Then there came after them their successors who said whatever they did not practise, and practised whatever they were not commanded to do. He who strove against them with his hand was a believer: he who strove against them with his tongue was a believer, and he who strove against them with his heart was a believer and beyond that there is no faith even to the extent of a mustard seed. Abu Rafi' said: I narrated this hadith to 'Abdullah b. 'Umar; he contradicted me. There happened to come 'Abdullah b. Mas'ud who stayed at Qanat, and 'Abdullah b 'Umar wanted me to accompany him for visiting him (as 'Abdullah b. Mas'ud was ailing), so I went along with him and as we sat (before him) I asked Ibn Mas'ud about this hadith. He narrated it in the same way as I narrated it to Ibn 'Umar.