Musnad Ahmad 32:2048

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard 'Abd al-'Aziz ibn Suhayb, he said: I heard Anas ibn Malik narrating from the Prophet, may Allah bless him and grant him peace, that he said: "Whoever wears silk in the world, he shall never wear it in the Hereafter."