It is narrated that Imam Mahdi Ahs said "if a spark is thrown into a house full of cotton, and is taken away instantly, it burns just a speck, but if the kalima, La ilaha illalah, stays at the heart of a person as long as it took for the spark to burn the speck, it (the kalima) would burn all the loves and affections of things other than Allah in the heart of that person."
Alas! Such an incident has never occurred when the kalima stayed at our heart even for such an insignificantly short period, in our long lives.
(Insaaf Nama - Matan Sharif, N.19)