He imagines that he is having intercourse with someone other than his wife
Question

                What is the ruling if, whilst having intercourse with his wife, a man imagines that she is some other woman and gets pleasure from that?.
Praise be to Allah.
    What this man is doing is something that the scholars have 
    stated is haraam, because even if he is only having intercourse with his 
    wife, he is imagining himself committing zina and feeling pleasure as a 
    result. 

    Ibn al-Haaj al-Maaliki (may Allaah have mercy on him) said: 

    A man should stay away from such thoughts and from this 
    abhorrent characteristic which unfortunately is widespread, which is that 
    when a man sees a woman he likes, he goes to his wife but he imagines that 
    woman whom he has seen. This is a kind of zina, because our scholars said 
    concerning one who takes a glass of water and imagines that it is wine that 
    he is drinking, that that water becomes haraam for him. What is mentioned 
    does not apply only to men, rather women are included in it too and that is 
    even worse, because usually women nowadays go out and look at people, and if 
    a woman sees someone whom she likes, she may keep thinking of him, then when 
    her husband has intercourse with her she imagines that image which she has 
    seen, so both of them may come under the heading of zina. We ask Allaah to 
    keep us safe and sound. End quote. 

    Al-Madkhal, 2/194, 195 

    Ibn Muflih al-Hanbali (may Allaah have mercy on him) said: 

    Ibn ‘Aqeel said – and he emphasized the prohibition in 
    al-Ri’aayah al-Kubra: If, when he is having intercourse with his wife, 
    he imagines the image of another woman or a male, then he is sinning. End 
    quote. 

    Al-Furoo’, 3/51 

    Wali al-Deen al-‘Iraaqi (may 
    Allaah have mercy on him) said: 

    If he has intercourse with his wife but in his mind he is 
    having intercourse with someone who is forbidden for him, and he imagines in 
    his mind that he is having intercourse with that forbidden image, that is 
    haraam for him, and that is because he is imagining himself committing a 
    haraam action. End quote. 

    Tarh al-Tathreeb, 2/19 

    And Allaah knows best.