Sa'id bin al-Musayyib (may Allah have mercy on him) said: "And upon those who are able to fast (only with hardship) is a ransom: the feeding of a poor person" [Surah al-Baqarah: 184]. This refers to the old man who used to fast and then became unable, and the pregnant woman for whom it is a hardship; upon them both is the feeding of one poor person for each day until the month of Ramadan is completed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8323