Musnad Ahmad 28:220

Yazid narrated to us, Muhammad ibn Ishaq informed us, from ‘Amr ibn Shu‘ayb, from his father, from his grandfather, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to teach us words to say when sleeping, in case of fright: "In the name of Allah. I seek refuge in the perfect words of Allah from His anger, His punishment, the evil of His servants, the incitements of the devils, and that they should attend me." He said: ‘Abdullah ibn ‘Amr used to teach them to those of his children who had reached the age of understanding to say them when they slept, and for those among them who were young and could not memorize them, he would write them down for him and hang it around his neck.