Musnad Ahmad 31:379

Yunus narrated to us, Layth narrated to us, from Ibn ‘Ajlan, from Sayfi Abu Sa‘id, the freed slave of the Ansar, from Abu al-Sa’ib, who said: I came to Abu Sa‘id al-Khudri, and while I was sitting with him, I heard the movement of something beneath his bed. I looked, and it was a snake, so I stood up. Abu Sa‘id said: "What is the matter with you?" I said: "A snake is here." He said: "And what do you want?" I said: "I want to kill it." He pointed for me to a room in his house opposite his own, and said: "A cousin of mine was in this room. When it was the Day of the Confederates (al-Ahzab), he asked the Messenger of Allah (peace and blessings of Allah be upon him) for permission to go to his family, as he was a newlywed. He gave him permission and ordered him to take his weapon with him. He came to his house and found his wife standing at the door of the house. He gestured toward her with his spear, and she said: 'Do not be hasty until you see what has brought me out.' He entered the house and found a monstrous snake, so he stabbed it with the spear. Then he went out with it on the spear, and it was writhing." He said: "I do not know which of them died faster, the man or the snake." His people came to the Messenger of Allah (peace and blessings of Allah be upon him) and said: "Supplicate to Allah to bring back our companion." He said: "Seek forgiveness for your companion," twice. Then he said: "Indeed, a group of the Jinn have accepted Islam. So if you see any of them, warn them three times. Then, if it appears to you after that to kill it, kill it after the third time."