Musannaf Ibn Abi Shayba 1:1805

Waki’ narrated to us, he said: Al-A’mash narrated to us, he said: I heard Mujahid say: The Messenger of Allah (peace be upon him) found the coming of Gabriel to be slow, so he said: "How can we come to you when you do not clip your nails, nor do you clean your knuckles, nor do you use the tooth stick (siwak)?"