Actions are by Intentions
‘Umar ibn al-Khattab radhiAllahu ‘anhu said that I heard the Prophet salAllahu ‘alayhi wa sallam say that actions are according to intentions. Each person will receive the reward according to his intention. He who migrated for a worldly gain he will only get the world (and will not get the reward of migration). And he who migrated to marry a woman then he will only get a woman (and will not get the reward of migration). [Bukhari]
No matter what a person does, even if it is a good deed, it is not done with the right intention then it will not benefit him. The intention for each action should be seeking the pleasure of Allah subhanahu wa ta’ala and keeping one’s Hereafter in mind.
Fasting to Show Off is Shirk
Shaddad ibn Aws radhiAllahu ‘anhu said that I heard the Prophet salAllahu ‘alayhi wa sallam say, “Whoever fasts to show off then he has committed shirk, and whoever performs Salah to show off, then he has committed shirk, and whoever gives charity to show off then he has committed shirk.” [Ahmad]
Fasting is by the Intention of the Heart
There are no words proven by the Sunnah of the Prophet salAllahu ‘alayhi wa sallam that are to be recited before eating the Sahoor. Recite Bismillah (in the Name of Allah) and begin eating.
This is a time for du’as to be answered, therefore, make a lot of personal du’as for yourself and your loved ones, especially, make du’as for forgiveness. It was narrated from Abu Huraira radhiAllahi ‘anhu that the Messenger of Allah salAllahu ‘alayhi wa sallam said, “Our Lord, may He be blessed and exalted, comes down to the lowest heaven every night when the last third of the night is left, and He says: Who will call upon Me that I may answer him? Who will ask of Me that I may give him? Who will ask Me for forgiveness that I may forgive him?” [Bukhari, Muslim]
Intention for the Obligatory Fast must be made before Fajr
Hafsa bint ‘Umar radhiAllahu ‘anha narrated that the Prophet salAllahu ‘alayhi wa sallam said, “Whoever does not intend to fast before dawn, his fast does not count.” [Abu Dawoud, at-Tirmidhi]
Intention for Voluntary Fast can be made any time before Noon
Voluntary Fast can be ended at Any Time
‘Aisha radhiAllahu ‘anha reported that one day the Prophet salAllahu ‘alayhi wa sallam came to my house and asked, “Do you have anything (to eat)?” I said, “No.” The Prophet salAllahu ‘alayhi wa sallam replied, “Then, I am fasting.” On another day, he again came to my house and I said, “O Messenger of Allah! Hais (a compound of dates and clarified butter) has been gifted to us.” He said, “Bring that. I had been fasting since the morning.” The Prophet salAllahu ‘alayhi wa sallam then ate it. [Muslim]
This indicates that a voluntary fast can be broken at any time. For example, you visit someone and the host offers you something to eat. You do not want to disclose you are fasting or you are a person who keeps voluntary fasts then you are allowed to break your fast and eat it. However, this is not applicable to obligatory fasts.