Hadith 4:221
Mu'tamir narrated to us, from Yunus, from al-Hasan, regarding fasting: He has the choice [to fast or not] as long as he has not entered the morning as a fasting person. But if he enters the morning as a fasting person, then he must fast.
حدثنا معتمر عن يونس عن الحسن في الصوم يتخير ما لم يصبح صائما فإذا اصبح صائما صام