Yahya related to me from Malik from Ismail ibn Muhammad ibn Saidibn Abi Waqqas from a mawla of Amr ibn al-As or of Abdullah ibn Amribn al-As that the Messenger of Allah, may Allah bless him and granthim peace, said, "The prayer of one of you sitting down is only equalto half the prayer of one of you when he is standing."
App reference: Book 8, Hadith 20
USC-MSA web (English) reference: Book 8, Hadith 20
Arabic reference: Book 8, Hadith 309