Can you mention the source for this statement, who said it and the Arabic text?
“Whoever fasts Ramadan while telling himself that when Ramadan ends, he will disobey his Rabb, his fast will be rejected.”
Imam Ibn Rajab Al Hambali (rahimahullah) has cited this as the statement of Ka’b Al-Ahbar. I have not come across this narration in any primary source.
(Lataiful Ma’arif, pg. 378, Al Maktabul Islami edition)
Arabic text of the part in question
من صام رمضان وهو يحدّث نفسه إذا أفطر بعد رمضان عصى ربّه فصيامه عليه مردود
Whoever fasts Ramadan and tells himself that when Ramadan ends he will disobey his Rabb, his fasting will be rejected.
And Allah Ta’ala Knows best.
Answered by: Moulana Suhail Motala
Approved by: Moulana Muhammad Abasoomar
This answer was collected from HadithAnswers.com. The answers were either answered or checked by Moulana Haroon Abasoomar (rahimahullah) who was a Shaykhul Hadith in South Africa, or by his son, Moulana Muhammad Abasoomer (hafizahullah), who is a Hadith specialist.