Sayyiduna Adi bin Hatim (may Allah be pleased with him) narrates: Whoever leads us in prayer should complete the bowing and prostration, for among us are the weak, the elderly, the sick, the traveller and people with work to attend to. This is how we used to pray behind the Prophet (peace and blessings be upon him).
Ahmad recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2369