If a person does not firmly resolve to fast from the night before, then he should not fast.
أخبرنا محمد بن عبد الأعلى قال حدثنا المعتمر قال سمعت عبيد الله عن نافع
إذا لم يجمع الرجل الصوم من الليل فلا يصم
No references are available.
Its chain of transmission is sahih, as a stopped narration. It is also found in Sunan al-Kubra, hadith no. 2652. Ubaydullah is Ibn Umar, and Mutamir is Ibn Sulayman.