In Book, Volume 2, pages 396-397 of Majmu'a Fatawa, Shaikh al-Islam Ibn Taymiyya speaks about fana also known in Sufism as annihilation.
"This state of love is the state of many people that are from the people of Love to Allah `azza wa jall, they are the people of the love of Allah and the People of the Will (al-irada) of Allah, it is typical of many of the people that love God and seek Him. Because that person has vanished in his lover, in Allah `azza wa jall Դhrough the intensity of the love, because he vanished in Allah's love, not his own ego's love. And he will recall Allah, not recalling himself, remember Allah , not remembering himself, visualizing Allah [yastashhid], not visualizing himself, existing in Allah, not in the existence of himself. When he reaches that state 'Ana al-Haqq' (I am the Truth) or 'Subhanee' (Glory to Me!) and he will say 'maa fil jubba ill-Allah' (there is nothing in this cloak except Allah), because he is drunk in the love of God and this is a pleasure and happiness that he cannot control."
Further on Shaikh al-Islam Ibn Taymiyya says:
"This [matter] has in it Haqq and there is in it Batil. But when someone will enter a state with his fervor intense love (`ishq) to Allah, he will enter a state of absentmindedness, and when he enters a state of absentmindedness, he will find himself as if he is accepting the [concept] of ittihad [union]. I do not consider this a sin. Because that person is excused and no one may punish him as he is not aware of what he is doing. Because the pen does not condemn the crazy except when he is restored to sanity. And when that person is in that state and he was wrong in what he did, he will be under Allah's address:
"Rabbana laa tu`akhidhna in-naseena aw akhta`na"
"Our Lord, do not take us to task if we forget or make mistakes." (Baqara 2:286)
"And Allah says in other verse, "wa laa junaaha `alaykum fimaa akhtaatum bihi" - "there is no blame on you if you unintentionally do a mistake."
On page 339, in Volume 10, Ibn Taymiyya says:
"there is a story of two men who were so respectful and loved each other very much. One of them fell in the water [of the sea] and immediately the other threw himself behind him. Then the first one, who was sinking asked, "what made you throw yourself here?" He said, I vanished in you, and when I vanished in you I thought you were me and I was you."
And further on Ibn Taymiyya continues:
"As long as he is not through something that is prohibited, it is accepted, but if it were prohibited (the intention was bad then he is not excused."
And Shaikh al-Islam Ibn Taymiyya continues (Volume 2, page 397):
"And because of that [situation]many of the saints like `Abdul Qadir Jilani, have an excuse because they are in a state of love `ishq)."
That subject is also mentioned in a whole chapter on detail from page 337-343, entitled: "al-Fana' alladhee yujad fi kalam as-sufiyya yuffassar bi-thalathat umur" (The Word Annihilation found in Sufism explained in Three Ways). This chapter describes the concept of fana' in detail.