Sahih Ibn Khuzaymah

Sayyiduna Imran bin Husain رضي الله عنه narrates from the Noble Prophet صلى الله عليه وسلم that he صلى الله عليه وسلم said: "The reward and merit of the one who prays lying down is half of that of the one who prays sitting." Imam Abu Bakr رحمه الله says that I have already explained that the Arabs use the word "na'im" (the one who sleeps) for a person who is lying down as well as for the one whose intellect and consciousness have been lost due to sleep. Indeed, in the statement of Mustafa صلى الله عليه وسلم, "the prayer of the one who sleeps," what is meant is the one who is lying down, not the one whose intellect and consciousness have been lost due to sleep. Because the person whose intellect has been lost due to sleep does not understand the prayer in that state (so how would he perform it?).