He may intend to fast the voluntary fast from the night until just before the time of the major forenoon prayer, as long as there is nothing that contradicts fasting after the dawn has broken. However, if something that contradicts fasting, such as eating, drinking, or sexual relations, occurs before that, whether intentionally or forgetfully, then the intention is not permissible thereafter, as stated in the Indian Fatwas 1: 196. It was narrated from Aisha, may Allah be pleased with her, that she said: "Whenever the Messenger of Allah, peace be upon him, entered upon me, he would say: 'Is there any food with you?' If we said: 'No,' he would say: 'I am fasting.' - Waki' added - One day he entered upon us, and we said: 'O Messenger of Allah, we have been gifted some 'Hais' - Hais is dates mixed with ghee and dried curd - so we saved it for you.' He said: 'Bring it closer to me.' Talha said: So he woke up fasting and broke his fast." This is found in Sunan Abu Dawood 2: 329, Sunan an-Nasa'i 2: 116, Al-Mujtaba 4: 195, Al-Mu'jam Al-Awsat 7: 233, and it was authenticated by Al-Suyuti in Al-Jami' Al-Saghir 1: 140. And from Umm al-Darda, may Allah be pleased with her: Abu al-Darda would say: 'Do you have food?' If we said: 'No,' he would say: 'Then I am fasting today.' Abu Talha, Abu Huraira, and Ibn Abbas, may Allah be pleased with them, did the same, as mentioned in Sahih al-Bukhari 2: 679, and Takhreej Al-Talaq 3: 144.