Hadith.2925 - Amr ibn Sa’id al-Sabati wrote to Abu Ja’far (as) asking about a man who had instructed in his will that Hajj be performed on his behalf by three different individuals. He asked if it was permissible for him to take one of those Hajj allocations for himself.
Imam (as) wrote back in his handwriting, and I read it:
"Perform Hajj on his behalf, God willing. You will receive a reward equal to his, and nothing will be diminished from his reward, God willing."