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." (FR)
English reference (FR): Vol. 1, Book 7, Hadith 1700 (FR)
Arabic reference (FR): Book 7, Hadith 1770 (FR)