It wasnarrated from Hafsah that the Messenger of Allah (ﷺ) said:
"Thereis no fast for the one who did not make it incumbent uponhimselffrom the night before." (PT)
English reference (PT): Vol. 1, Book 7, Hadith 1700 (PT)
Arabic reference (PT): Book 7, Hadith 1770 (PT)