Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) says: When this verse was revealed: "And upon those who are able to (fast) is a ransom: the feeding of a poor person" [Surah al-Baqarah: 184], whoever among us wished would fast, and whoever wished would not fast and would give the ransom; then after it this verse was revealed: "So whoever of you witnesses the month, let him fast it" [Surah al-Baqarah: 185].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7897