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