Abu Dawud Book 7, Hadith Number 2311.

Narated By Abdullah ibn Abbas : Explaining the verse; “For those who can do it (with hard-ship) is a ransom, the feeding of one, that is indigent,” he said: This was a concession granted to the aged man and woman who were able to keep fast; they were allowed to leave the fast and instead feed an indigent person for each fast; (and a concession) to pregnant and suckling woman when they apprehended harm (to themselves).