The Messenger of Allah ﷺ said, "Whoever does not form the intention to fast before dawn, there is no fast for him."
حدثنا أحمد بن صالح حدثنا عبد الله بن وهب حدثني ابن لهيعة ويحيى بن أيوب عن عبد الله بن أبي بكر بن حزم عن ابن شهاب عن سالم بن عبد الله عن أبيه
أن رسول الله ﷺ قال من لم يجمع الصيام قبل الفجر فلا صيام له
[Its chain is daif] Tirmidhi recorded it, Sawm, chapter, "There is no fast for one who does not decide the night before," no. 730, and Nasai, no. 2333, from the hadith of Yahya bin Ayub, and Ibn Majah, no. 1700, from the hadith of Abdullah bin Abu Bakr, through it, and Tirmidhi said, "Gharib." Zuhri used ambiguous transmission wording (anana).