Mujahid (may Allah have mercy on him) narrates that Sayyiduna Ibn Abbas (may Allah be pleased with them both) used to recite: "And upon those who are burdened with it (yutawwaqunahu)" [Surah al-Baqarah: 184]. He would say: This refers to the very old man who is not able to fast, so he breaks his fast and should feed a poor person half a sa' of wheat in place of each day.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8317