Ahmad ibn Muhammad from his father from Abdullah ibn Al-Mughira from Abdullah ibn Bukayr:
Abu Abdillah عليه السلام said: "The tashahhud in the supererogatory prayer is part of the tashahhud of the obligatory prayer."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 340