It is narrated from the wife of the Messenger, Sayyida Hafsa (may Allah be pleased with her), that the Noble Prophet (peace and blessings be upon him) said: Whoever does not make the intention to fast before Fajr, there is no fast for him.
Hadith Referenceالفتح الربانی / كتاب الصيام / 3703
Hadith Gradingمحدثین:ضعیف
Hadith Takhrij«اسناده ضعيف، فيه ابن لھيعة سييء الحفظ، ثم انه اختلف عليه۔ اخرجه ابوداود: 2454، والترمذي: 730، والنسائي: 4/ 196، وابن ماجه: 1700، (انظر مسند أحمد ترقيم الرسالة: 26457 ترقیم بيت الأفكار الدولية: 26989»
Brief Explanation
Benefits: … In Abu Dawood and Tirmidhi, instead of "ma‘a al-fajr" (with the dawn), the wording is "qabla al-fajr" (before the dawn). The meaning of this hadith is that if a person does not make the intention (niyyah) for fasting before the break of dawn (fajr), his fast will not be valid. However, from the following narrations and their explanations, it will become clear that this hadith pertains to a person who intends to keep an obligatory (fard) fast, is aware of this fast, and is also awake. Furthermore, you should carefully study the following narrations and their explanations.