Hadith.1164 - Ubaydullah ibn Ali Al-Halabi narrated from Zurarah from Imam Jafar ibn Muhammad Al-Sadiq (as) who said
I asked him about a person who is behind the Imam, and the Imam prolongs the tashahhud (sitting for testimony).
Imam (as) said: "He may perform the taslim (salutation of peace) and leave if he wishes."