The obligation of the fasting person if his fast is obligatory is that he refrains from food and drink and the other things that break the fast after verifying the entry of the time of Fajr himself. He may also verify the entry of Fajr by hearing the Athan of the caller who normally calls the Athan after Fajr comes or he may base his verification on charts that list the times of the entry of Fajr. This is due to Allah's Statement: "And eat and drink until the white thread of dawn becomes distinct to you from the black thread. Then complete the fast until night" (Al-Baqarah 2: 187).
This is also due to the Prophet's statement: “Verily, Bilal calls the Athan at night so eat and drink until Ibn Umm Maktum makes the call” (Al-Bukhari no. 617).
He (Ibn Umm Maktum) was a blind man who would not give the call to prayer until it was said to him, "Morning has come, morning has come." This narration is agreed upon (in Al-Bukhari and Muslim). Thus, if a person eats after that or drinks or does anything that would break the fast, his fast is nullified. In reference to one who is fasting a voluntary fast, his fast is not complete unless he refrains from eating, drinking and the other things that break the fast from the time when Fajr time enters, just like the one who is fasting an obligatory fast. If he eats or drinks or does anything that would break the fast after Fajr time enters or after the Athan that is made at the time of Fajr's entry, then he has no fast. However, the person fasting a voluntary fast differs from the person fasting a voluntary fast in that it is permissible for him to start his fast during the daytime if he did not do anything that would break the fast after Fajr time began. The reward for fasting is written for him from the time that he had intention to fast. This is due to the statement of 'A'ishah: “The Messenger of Allah (salallahou 'aleyhi wa salam) entered upon us one day and said: 'Do you all have anything (to eat)?' We replied, 'No.' Then he said: The I am fasting.' Then he came to us on another day and we said: 'We have been given some Hais (a mixture of dates and clarified butter) as a gift.' So, he said: “Show it to me, for verily I have been fasting since morning' (Muslim no. 1154). Then he ate it." This was recorded by Muslim. Then, there is also his statement: "Actions are only with intentions, and every man will only get what he intended" (Al-Bukhari no. 1 and Muslim no. 1907).