It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The prayer of one who sits is equal to half the prayer of one who stands, and the prayer of one who lies down is equal to half the prayer of one who sits."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15681