Imam in surah isra ayah 71 refers to the Quran.
Surah Ra'd ayah 7. The full ayah is this
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YUSUFALI: And the Unbelievers say: "Why is not a sign sent down to him from his Lord?" But thou art truly a warner, and to every people a guide.
PICKTHAL: Those who disbelieve say: If only some portent were sent down upon him from his Lord! Thou art a warner only, and for every folk a guide.
SHAKIR: And those who disbelieve say: Why has not a sign been sent down upon him from his Lord? You are only a warner and (there is) a guide for every people.
the guide refers to the prophet Muhammad (saw) and other prophets that were sent before him.
