What is said when going to sleep in Islam instead of "Good night"?

No specific utterance is mandated when going to sleep. However, it is recommended for someone who wishes to go to sleep and leave the gathering of his household to bid them farewell with "salam," based on the general hadith narrated by Abu Hurairah (may Allah be pleased with him), who said that the Messenger of Allah (peace be upon him) said: "When one of you comes to a gathering, let him greet them with salam; and when he wishes to stand up, let him greet them with salam. The first is no more deserving than the last."

There is no harm in saying "Tusbihoona 'ala khair" (May you wake up to goodness) or similar phrases, along with the salam, but these should not replace the prescribed salam.