Muhammad ibn Ali ibn Mahbub from Ali ibn al-Sindi from Safwan from Ishaq ibn Ammar from Abu Abdillah عليه السلام, peace be upon him, who said:
Imam al-Sadiq عليه السلام said: “It is proper for the imam (of prayer) that his prayer be according to the weakest among those behind him.” (meaning that the prayer leader keep the prayer of suitable length according to those following him)
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 3, p. 302