Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: One night I stood to pray with the Prophet (peace and blessings be upon him). I stood on his left side, so he (peace and blessings be upon him) reached his hand round behind him, took hold of my hand, or my upper arm (the narrator is in doubt), and placed me on his right side.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12514