Ahmad ibn Al-Hasan from Amr ibn Sa'id from Musaddiq ibn Sadaqah from Ammar ibn Musa Al-Sabati, who said:
I asked Abu Abdillah عليه السلام, about the taslim, what is it? Imam عليه السلام said: "It is permission (to end the prayer)."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 341