الكافي 2/1/42/4

4. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad ibn ‘Isa from Shadhan ibn al-Khalil. The narrator has said that he wrote down from his book through the chain of his narrators who narrated in a marfu’ manner from ‘Isa ibn ‘Abd Allah saying: ‘“Isa ibn ‘Abd Allah once said to Abu ‘Abdillah عليه السلام ‘May Allah keep my soul in service for your cause, what is worship?’ The Imam said, ‘It is good intention in obedience to Allah concerning the aspects through which He is obeyed. As for you, O ‘Isa, you will not be a believer until you can distinguish the abrogating from the abrogated.’ I then said, ‘May Allah keep my soul in service for your cause, what is distinguishing the abrogating from the abrogated?’ The Imam said, ‘Is it not true that you obey an Imam with good intention and in good faith, and when that Imam passes away and another Imam comes, you settle your soul on obedience to him with good intention and in good faith?’ I said, ‘Yes, I do so.’ The Imam said, ‘This is distinguishing the abrogating from the abrogated.’”