Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Do you know how Islam decreases?" The people said: As the dye of a garment fades, as an animal (weakens and) diminishes, and as a dirham becomes rough from being kept a long time. He said: "That too is part of it (the decrease), but greater than that is the death of the scholars, or their departure."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8959