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Shiavault - a Vault of Shia Islamic Books Hayat Al-Qulub Vol.3 Part 5: The one who denies one Imam is like the one who denies all Imams Kulaini and others have, with reliable chains of narrators, narrated from Imam Sadiq (a.s.) that: One who denied one Imam has denied all Imams.
Ibne Babawayh, Nomani and others have, with reliable chains of narrators, narrated from Aban bin Uqaab that: I asked Imam Sadiq (a.s.) is he a believer who knows all the past Imams but denies (does not recognize) the Imam of his own time? He replied: No. I asked: Is he a Muslim? He said: Yes. Ibne Babawayh says that Islam means confession of three facts whereby his life and property become secure and the reward of the Hereafter depends on Faith.
The Holy Prophet (S) has said: The one who testified to the Oneness of Allah and my Prophethood has secured his life and property, (from being executed or confiscation of property) and his accountability is with Allah on the Day of Judgement. He has also narrated from Imam Sadiq (a.s.) that: You should know that if someone denies the Messengership of Isa (a.s.) and believes in the Prophethood of all other messengers is not a believer.
Seek the path of Allah by search and want of that Imam whose sign is Haqq (truth) and when your Imam is hidden, search for the signs, writings and words of the Imams, which are before you and perfect your religious affairs. Then you will have believed in your Lord. He has narrated with authentic chains of narrators from Imam Ridha (a.s.) that the Holy Prophet (S) said: O Ali!
You and the Imams from among your sons are, after me, the Hujjat (proofs) of Allah over the creation and they are the signs of the path of Allah for the people. The one who denied anyone of them denied me and who disobeyed anyone of them disobeyed me and the one who oppressed any of them has oppressed me and the one who behaved nicely and righteously with them has behaved, loved fully and affectionately with me.
Whoever obeyed you has obeyed me who befriended you has befriended me and who became your enemy has become my enemy because you are born of me and my loins and I am from you. Nomani has narrated from Bin Numaa, that I said to Imam Ridha (a.s.) that such and such person is your slave and a Shia and sends salutation (Salaam) to your honour and says: You please be my surety in Qiyamat and assure that you will intercede in my favor. The Hazrat asked: Is he from our Shias and friends? I said: Yes.