Tafsir of At-Tawbah 9:125

"As for those in whose hearts is a disease," meaning hypocrisy, "it only added filth to their filth," meaning hypocrisy added to their hypocrisy. The word "added" implies the meaning of joining or appending, which is why it is connected with the preposition ila (to). It has been said that ila here carries the meaning of ma'a (with), though there is no need for such an interpretation. "And they died while they were disbelievers," meaning that this state became firmly established in them until they died upon it.