Abu Hurairah (radiyallaahu-anhu) narrated that the Prophet (sallal-laahu-alayhi-wasallam) said: ”It may be that a fasting person’s share of the fasting is nothing else but hunger and thirst.” [Imaam Ibn Maajah: 1690; Imaam Ahmad: 8639 and declared authentic by Imaam Albaanee in Saheeh Al-Jaami 3488, 3490]

Imaam Ibnul Qayyim (rahimahullaah) said:

Fasting is abstinence of the limbs from sins and abstinence of the stomach from food and drink. And just as food and drink nullifies and corrupts the fasting, likewise sins curtail its reward and corrupt its fruits, so it leads him (i.e. the person) to the position of one who has not fasted. [Sharh Waabilus Sayyib of Imaam Abdul Azeez Bin Baaz, (rahimahullaah), page: 32]