He was praying and placed his left hand over his right. The Prophet ﷺ saw him and placed his right hand over his left.
حدثنا محمد بن بكار بن الريان عن هشيم بن بشير عن الحجاج بن أبي زينب عن أبي عثمان النهدي
أنه كان يصلي فوضع يده اليسرى على اليمنى فرآه النبي ﷺ فوضع يده اليمنى على اليسرى
[Its isnad is hasan] Nasai narrated it in the Book of Opening of Prayer, chapter on the imam when he sees a man placing his left hand over his right, hadith 889, and Ibn Majah, hadith 811, from the hadith of Hushaym, with the same wording, and he stated explicitly that he heard it directly.