Abdullah bin Amr (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) said: "The reward of one who prays sitting is half that of one who prays standing."
(b) Jarir narrates from Mansur that praying sitting was a special characteristic of the Prophet (peace and blessings be upon him); and the meaning of his (peace and blessings be upon him) statement: "Praying sitting brings half the reward of one praying standing," will come in the chapter of special characteristics at the beginning of the Book of Marriage (Kitab al-Nikah), if Allah wills.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4593