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qul ʾayyu shayʾin ʾakbaru shahādatan quli llāhu shahīdun baynī wa-baynakum wa-ʾūḥiya ʾilayya hādhā l-qurʾānu li-ʾundhirakum bihī wa-man balagha ʾa-ʾinnakum la-tashhadūna ʾanna maʿa llāhi ʾālihatan ʾukhrā qul lā ʾashhadu qul ʾinnamā huwa ʾilāhun wāḥidun wa-ʾinnanī barīʾun mimmā tushrikūna
Say, ‘What thing is greatest as witness?’ Say, ‘Allah! [He is] witness between me and you, and this Quran has been revealed to me that I may warn thereby you and whomever it may reach.’ ‘Do you indeed bear witness that there are other gods besides Allah?’ Say, ‘I do not bear witness [to any such thing].’ Say, ‘Indeed He is the One God, and I indeed disown what you associate [with Him].’
— Ali Quli Qarai
Surah Al-‘An’am, Verse 19
19. "Say: 'What thing is the greatest in testimony?' Say: ' Allah ! He is witness between me and you, and this Qur'an has been revealed to me that I may warn you thereby, and whomever it reaches. Do you indeed testify that there are other gods with Allah ? ' Say: ' I do not testify '. Say:, He is only One God, and verily I am quit of that which you associate (with Him) '."
The pagans of Mecca demanded witness from the Prophet (S) for his prophet hood. They did not accept the prophecy of the Messenger of Allah (S) and told him that even the Jews and the Christians did not know him a divine prophet. This holy verse, with an atmosphere of Allah’s inspiration and help, and at the time of loneliness of Islam, foretells a bright and promising future for Muslims, when it hates polytheism. For three times, and in a short phrase, the Qur'an refers to Unity of Allah while quitting polytheism. The verse says:
Besides all other prophetic miracles, Unseen aids, and frustrating the hostile plots, the Qur'an itself is the greatest evidence to the Prophet's Messenger ship. The messenger ship of Muhammad (S) is both worldly and eternal, for all humankind and in all ages. However, a permanent warning indeed, should be accompanied with an everlasting warner. Therefore, the divine Imamate and leadership should also be together with the Qur'an forever. This meaning is cited in Tafsir-us-Safi and Usul-i-Kafi, narrated from Iman Sadiq (as)