Sayyiduna Abdullah bin Umar narrates this saying of the Prophet (peace and blessings be upon him): "The prayer of one who prays sitting is half the prayer of one who prays standing (in terms of reward)."
Al-Bazzar and al-Tabarani in al-Mu'jam al-Kabir recorded it, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2902