Fana (annihilation) and baqa (subsistence, abiding) are the paired terms in which Sufi writers describe the goal of the path: the passing away of the self's own attributes, willing and awareness, and the abiding that is said to follow it. The Qur'anic anchor is 55:26-27, "everyone upon it passes away, and there remains the Face of your Lord, possessor of majesty and honour", read as a statement about the structure of reality that the traveller comes to realise rather than merely to believe.
The vocabulary took shape in the ninth and tenth centuries. Abu Yazid al-Bistami (d. c. 874) is the figure associated with its most extreme expressions; al-Junayd of Baghdad (d. 910) gave it the formulation that became normative in sober Sufism, describing fana as a return to the condition of the primordial covenant of 7:172 and insisting that it does not abolish the distinction between servant and Lord. Junayd's reading matters because it is the one that let Sufism be defended as orthodox.
The ecstatic utterances known as shathiyyat are where the dispute concentrates. Al-Bistami's "Glory be to me, how great is my majesty" and al-Hallaj's "ana al-haqq", I am the Real, were read by sympathisers as speech uttered in a state where the speaker is not present to himself, and by others as blasphemy; al-Hallaj was executed at Baghdad in 922 in a trial where this was among the charges, though the political dimensions of the case were considerable. Later metaphysics named two rival ways of construing what fana discloses: Ibn Arabi's wahdat al-wujud, the oneness of being, taken by critics to erase the creator-creature distinction, and Ahmad Sirhindi's wahdat al-shuhud, the oneness of witnessing, which holds that the mystic perceives nothing but God without it following that nothing but God exists. Both positions have had learned defenders and both have been charged with the other's errors.