Salama b. Akwa’ reported: We, during the lifetime of the Messenger of Allah (ﷺ), in one month of Ramadan (observed fast according to our liking). He who wished to fast lasted and he who wished to break it, broke it and fed a needy person as an expiation till this verse was revealed:” He who witnesses among you the month (of Ramadan) he should observe fast during it” (Al-Baqarah 185).
حَدَّثَنِي عَمْرُو بْنُ سَوَّادٍ الْعَامِرِيُّ، أَخْبَرَنَا عَبْدُ اللَّهِ بْنُ وَهْبٍ، أَخْبَرَنَا عَمْرُو بْنُ، الْحَارِثِ عَنْ بُكَيْرِ بْنِ الأَشَجِّ، عَنْ يَزِيدَ، مَوْلَى سَلَمَةَ بْنِ الأَكْوَعِ عَنْ سَلَمَةَ بْنِ الأَكْوَعِ، – رضى الله عنه – أَنَّهُ قَالَ كُنَّا فِي رَمَضَانَ عَلَى عَهْدِ رَسُولِ اللَّهِ صلى الله عليه وسلم مَنْ شَاءَ صَامَ وَمَنْ شَاءَ أَفْطَرَ فَافْتَدَى بِطَعَامِ مِسْكِينٍ حَتَّى أُنْزِلَتْ هَذِهِ الآيَةُ { فَمَنْ شَهِدَ مِنْكُمُ الشَّهْرَ فَلْيَصُمْهُ}
Reference : Sahih Muslim 1145b
In-book reference : Book 13, Hadith 192
USC-MSA web (English) reference : Book 6, Hadith 2548