Musnad Abu Ya'la al-Mawsili 151:75

Surayj ibn Yunus narrated to us, Hushaym narrated to us, from al-Hajjaj ibn Abi Zaynab, from Abu ‘Uthman, from ‘Abdullah, who said: I was praying and had placed my left hand upon my right. The Prophet, may Allah bless him and grant him peace, came to me and took my right hand and placed it upon my left.