Narrated by as-Saduq in Maanil Akhbar (p 104):
Musa ibn Jafar (a.s.) said, ‘Once when the Messenger of Allah was sitting an angel who had twenty four faces came to him. The Messenger of Allah said to him, “My friend Jibril, I had not seen you in this form before.” The angel said, ” Iam not Jibril, (I am Mahmud). Allah, the Most Majestic, the Most gracious, has sent me to join in marriage the light with the light.” He then asked, “Who with who?” The angel said, “Fatima and Ali (a.s.).” The Imam (a.s.) said, “When the angel turned back on his shoulder it said, “Muhammad the Messenger of Allah, and Ali (a.s.) the executor of his will.” The Messenger of Allah asked, “Since when this has been written on your shoulder?” The angel replied, “It was there twenty two thousand year before Allah created Adam.”
We all know that shias believe that neither prophet (sallalahu alaihi wa ala alihi wa sallam) nor Imams could err.
Majlisi in Rawda said hadith is strong (5/349), and also said strong as saheeh (8/187).