Imran bin Husayn (may Allah be pleased with him) said: I asked the Prophet (peace and blessings be upon him) about one who prays sitting, and he (peace and blessings be upon him) said: "Whoever prays standing, that is better; whoever prays sitting has half the reward of the one standing; and whoever prays lying down, his reward is half that of the one praying sitting."
(b) In the hadith of Ishaq: he asked the Prophet (peace and blessings be upon him) about one who prays sitting.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4592