Mukhtar says: I asked Sayyiduna Anas (may Allah be pleased with him) about the prayer of a sick person, and he said: He should perform the bowing and the prostrations while sitting in the obligatory prayer.
Ahmad recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2901