Musannaf Ibn Abi Shayba 3:285

Ibn ‘Uyaynah narrated to us, from Sumayy, from al-Nu‘man ibn Abi ‘Ayyash, who said: They complained to the Prophet (may Allah bless him and grant him peace) about leaning and supporting oneself in prayer. So he granted them a concession that a man may support himself with his elbows upon his knees or his thighs.