The great Mystic, Manur al Hallaj was martyred on 26 March, 922, partly because of the following saying that expressed the nature of his mystic union: ana'l –Haqq (meaning "I am Truth"). Mystic Mansur would go into trances often, intoxicated with Divine Love and during one of these trances, he would utter ana'l-Haqq, and also, "In my turban is wrapped nothing but God?" which was taken to mean that he was claiming to be God, as Al-Haqq is one of the Ninety Nine Names of Allah. In another statement, Hallaj would point to his cloak and say, "Maa Fil Jubbati Illa-Allah" meaning "There is nothing inside the cloak except God."
This utterance would lead him to a long trial, and subsequent imprisonment for eleven years in a Baghdad prison. In the end, he would be tortured and publicly crucified (in some accounts he was beheaded and his hands and feet were cut off) by the Abbasid rulers. Many accounts tell of Al-Hallaj's calm demeanor even while he was being torture and even he went to his execution dancing in his chains.
Here is a beautiful explanation of the statement "I am the Truth", (Anal Haqq), and the reality behind the utterance. It is quoted from spiritual-health website.
‘Ana’l Haqq’ is, in fact, entailed by, and an expression of, the central meaning of the first pillar of Islam in which one is required to bear witness that there is no reality but Divinity. To say there is 'no god but God’ (La ilaha illaAllah), does not get to the heart of the matter, for one must ask what is Divinity, and when one addresses this question in essential, fundamental terms, the answer to that inquiry is that God is the basis, and cause, of all manifested possibilities. In short, there is no reality but Divinity, and any statement to the contrary is in error.
All of Creation is nothing but a multiplicity of manifested permutations and combinations of Divine Names and Attributes brought together through Divine Purpose. There is no aspect of Creation whose nature, qualities, character, form, potential, possibilities, capacity, behavior, and duration are not a function of the manner in which Divinity arranges Names and Attributes in order to give expression to manifestation of one kind, rather than another.
Consequently, if the ‘I’ of "Ana’l Haq" - that is, ‘I am the Truth’, is a realized reference to the Divine nature of manifestation - human or otherwise, and if ‘Haq’, or Truth, is one of the Names of Divinity - which it is - then, in reality, the statement in question is actually an alternative way of stating the very same truth which is contained in the Shahadah - the declaration that there is no reality but Divinity. Although the public declaration of ‘Ana’l Haq’ constitutes a violation of Shari‘ah, nonetheless, the statement is only heretical or untruthful when it is said through a human being who is not in a realized spiritual condition** of tawhid."
** Indian Vedantic Philosophy very much stress this importance of self realization, because without self-realization of the truth, everything else is just theory. When Einstein was asked, "Do you believe in God", he replied, "No i do NOT believe in God." pausing he said, "I know there IS a God." To me this is a beautiful expression of realization within. compared to which, just believing without realization sounds very illusory.
Here is another very important saying of Hallaj which reflects the sincerity of his special heart, Allah, Most High, is the very One Who Himself affirms His Unity by the tongue of whomsoever of His creatures He wishes. If He affirms His Unity in my tongue it is He Who does so, and it is His Affair.
Otherwise, my brother, I myself have nothing to do with affirming Allah's Unity.