Al-Junayd of Baghdad (d. 910) systematized fana and baqa as the sober grammar of Sufi experience: the passing-away of human attributes and the subsistence in divine attributes. The caution was learned from al-Hallaj, executed in Baghdad in 922, whose utterance ana al-Haqq, 'I am the Truth', was taken as a claim of divinity. Louis Massignon's four-volume study, The Passion of al-Hallaj (1922), remains the monumental source; Ibn Arabi's wahdat al-wujud, unity of being, later gave the experience its metaphysical frame.
Modern psychedelic and New Age vocabulary borrows fana wholesale as 'ego death', usually without the baqa: the classical point is not the dissolution but the return, the annihilated one comes back to serve, which is why Junayd called sobriety after ecstasy the higher station.
In the refined doctrine, fana does not merge the drop into the ocean; it polishes the self until only the reflection remains. The one who says ana al-Haqq after full annihilation is, on this reading, not speaking, God speaks through the emptied place. The debate over whether that sentence is heresy or precision is the doctrine's live wire.
If the self must die for God to appear, who reports back?