The Messenger of Allah ﷺ said, "There was no prophet whom Allah sent to a nation before me except that he had from his nation devoted disciples and companions who followed his way and obeyed his command. Then there came after them generations who said what they did not do and did what they were not commanded. 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 so much as a mustard seed of faith."
أن رسول الله ﷺ قال: ما من نبي بعثه الله في أمة قبلي إلا كان له من أمته حواريون وأصحاب يأخذون بسنته، ويقتدون بأمره، ثم إنها تخلف من بعدهم خلوف، يقولون مالا يفعلون، ويفعلون مالا يؤمرون، فمن جاهدهم بيده فهو مؤمن، ومن جاهدهم بلسانه فهو مؤمن، ومن جاهدهم بقلبه فهو مؤمن. وليس وراء ذلك من الإيمان حبة خردل.
Muslim narrated it in the Book of Faith (50) through several routes from Yaqub bin Ibrahim bin Saad, who said, my father narrated to me, from Salih bin Kaysan, from Harith, from Jafar bin Abdullah bin Hakam, from Abdul-Rahman bin Miswar, from Abu Rafi, from Ibn Masud, and he mentioned it.