Musnad Abu Ya'la al-Mawsili 161:40

Abu Musa Muhammad ibn al-Muthanna narrated to us, Abd al-Wahhab narrated to us, Ubayd Allah narrated to us, from Suhayl, from his father, from Abu Hurayrah, that the Prophet (peace and blessings of Allah be upon him) missed a man from his companions. Then he met him and said, "What is the matter with me that I have not seen you?" He said, "I did not sleep last night; a scorpion stung me." He said, "Had you said when you entered the evening, 'I seek refuge in the perfect words of Allah from the evil of what He created,' it would not have harmed you." Ubayd Allah said, "And I do not know it except that he said in the hadith, attributing it to the Prophet (marfu'): 'Whoever says it when he enters the evening and when he enters the morning, it will not harm him.'"