Sayyidah Aishah Siddiqah (may Allah be pleased with her) narrates this as a marfu' hadith: The Prophet (peace and blessings be upon him) said: "The prayer of one who (prays) sitting is half the prayer of one who (prays) standing."
Ahmad recorded it, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2903