Ammar al-Sabati said:
I (narrator) asked Abu Abdillah عليه السلام, peace be upon him, about ‘the fasting person who is afflicted by thirst until he fears for himself (from death).’ Imam عليه السلام said: “He drinks the amount by which he preserves his remaining life, and he does not drink until he is quenched.”
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 4, p. 407