The Messenger of Allah (ﷺ) said: He who does not determine to fast before dawn does not fast.
Sunan Abi Dawud Book 14, Hadith 142
← Previous Hadith I asked 'Aishah: Would the Messenger of Allah (ﷺ) fast three days every month ? She replied: Yes. I asked: Which days in the month he used to fast ? She replied: He did not care which days of the month he fasted.

Sunan Abi Dawud Book 14, Hadith 141 Next Hadith → When the Prophet (ﷺ) entered upon me, he would ask: Do you have food ? When we said: No, he would say: I am fasting. Waki' added in his version: Another day when he entered upon us, we said: Messenger of Allah, some pudding (hair) has been presented to us and we have retained it for you. He said: Bring it to me. Talhah said: He fasted in the morning, but broke his fast (that day).

Sunan Abi Dawud Book 14, Hadith 143