Hasan, regarding the person who becomes unconscious while sitting, says that he should perform ablution.

Ibrahim says that when he regains consciousness, he should perform ablution.

Hazrat Ubaidullah bin Abdullah says that I went to Hazrat Aisha (may Allah be pleased with her) and requested, "Tell me about the illness of the Messenger of Allah (peace and blessings be upon him) at the time of his death." Hazrat Aisha (may Allah be pleased with her) said: When the Messenger of Allah (peace and blessings be upon him) fell ill, his condition became very heavy, then he fell unconscious. When he regained consciousness, he said, "Put water for me in a large vessel." So we put water in it and he performed ghusl (ritual bath). He got up with great difficulty, then he fell unconscious again. When he regained consciousness, he said, "Put water for me in a large vessel," and we did so. He again performed ghusl and got up with great difficulty, and again he fell unconscious. Then when he regained consciousness, he said, "Put water for me." So he again performed ghusl, and he did this several times.