Mujahid (may Allah have mercy on him) narrates: I heard Qays bin al-Sa'ib (may Allah be pleased with him) say that a person can give ransom for the month of Ramadan by having a poor person fed on his behalf for each day, "so feed two poor people on my behalf."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8322