The requirement for the fasting person to make the intention from the night for obligatory fasts, not voluntary ones

It is narrated from Sayyidah Maymunah bint Sa'd (may Allah be pleased with her) that she said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "Whoever makes a firm resolve (intention) to fast during the night, let him fast; and whoever reaches the morning without having made the intention during the night, let him not fast."
This ruling is for the obligatory fast, not for the voluntary fast.

Imam Musaddad (may Allah have mercy on him) narrated: Abd al-Wahid narrated to us, from Layth, from Mujahid, from one of the pure wives of the Prophet (peace and blessings be upon him) (may Allah be pleased with her), that she said: The Prophet (peace and blessings be upon him) would (sometimes) come (home) and ask about food, and when he found nothing in the house, he (peace and blessings be upon him) would make the intention to fast. She said: Sometimes it would happen that he (peace and blessings be upon him) came while he (peace and blessings be upon him) was fasting and I had some fine gift or some new food, and we would say: "O Messenger of Allah, if you were not fasting, we would offer you this food." Then he (peace and blessings be upon him) would ask for that food and eat it.
Explanation: According to the imams and commentators, this hadith is evidence that for a voluntary fast it is not a condition to make the intention before true dawn, and that it is also permissible in the Shari'ah to break it for some benefit or for hospitality.

Abu Sufyan (may Allah have mercy on him) states: I heard a man asking Sayyiduna Anas bin Malik (may Allah be pleased with him): "I had taken the pre-dawn meal (suhur), then I decided to break the fast; may I do so?" Sayyiduna Anas (may Allah be pleased with him) said: "Break it." Then he said: Sayyiduna Abu Talhah (may Allah be pleased with him) would come to his family and ask: "Do you have anything to eat?" When they said: No, he would say: "Then I am fasting."
Explanation: This report shows that the intention for a voluntary fast can also be made during the day, and also that there is room to break a voluntary fast after having begun it.

It is narrated from Sayyidah Umm al-Darda' (may Allah be pleased with her) that Sayyiduna Abu al-Darda' (may Allah be pleased with him) would come to the family in the morning and ask them about food, and when no food was available he would say: "Then I am fasting."
This report is sahih (authentic) as mawquf (a Companion's statement).