The Importance of Having Sincerity when Fasting
Al-Imaam Muqbil bin Haadi Al-Waadi’ee, may Allah have mercy upon him, says in his great book, Al-Jaami’ As-Saheeh Vol.2, Pge. 482, Hadeeth No. 1479
“Having Sincerity When Fasting
Upon the authority of Abu Hurayrah, May Allah be pleased with him that he said: the Messenger of Allah (may be peace upon him) said:
“It could be that the fortune of the one who is fasting is just hunger and thirst and it could be that the fortune of the one who is standing in prayer is just sleeplessness.”
This hadeeth is Hasan”
The hadeeth is reported by Al-Imaam Ahmad (v.2, pg 373) and also translated by meaning and without the chain of narration.