Maliks Muwatta Book 8, Hadith Number 20.
Section : The Excellence of the Prayer of Someone Standing over the Prayer of Someone Sitting.
Yahya related to me from Malik from Ismail ibn Muhammad ibn Said ibn Abi Waqqas from a mawla of Amr ibn al-As or of Abdullah ibn Amr ibn al-As that the Messenger of Allah, may Allah bless him and grant him peace, said, “The prayer of one of you sitting down is only equal to half the prayer of one of you when he is standing.”