the Prophet said: "Whoever did not decide to fast before Fajr then there is no fast for him."
Jami` at-Tirmidhi Book 8, Hadith 49
← Previous Hadith "The Messenger of Allah would kiss and fondle while he was fasting, and he had the most control among you of his limbs."

Jami` at-Tirmidhi Book 8, Hadith 48 Next Hadith → "I was sitting with the Prophet when some drink was brought, so he drank from it, then he offered it to me and I drank it. Then I said: 'I have indeed sinned, so seek forgiveness for me.' He said: 'What is that?' I said: 'I was fasting, then I broke the fast.' He said: 'Were you performing a fast that you had to make up?' I said: 'No.' He said: 'Then it is no harm for you.'"

Jami` at-Tirmidhi Book 8, Hadith 50