AS or alayhis sala'am is the Arabic pronunciation of alev ha shalom or May Peace be upon [the departed one]..
Arabic is the daughter language of Hebrew, just in case you don't know that, and the Arabs are the cousins of the Jews, since they belong to the 12 Tribes of Ishmael, the elder half brother of Isaac, from whose 12 Tribes, the Jews are descended.
However, that doesn't make all Muslims relatives of the Jews because more than 90% of the Muslims are not Arab.
Don't be surprised at the ignorance displayed by the Muslims since they are not Arab and don't know what you mean.
It is not found in the Koran, but, is used in the normal course as in the custom.
By the way, the yarmulke is the kufi cap for the Muslims, and the kipot is the hijab for the Muslims, the khitan is the bar mitzvah for the Muslims, the fast of Ashura was prolonged by Mohammad to make Ramadan, and the original qiblah of the Muslims was the mizrah of the Jews - Jerusalem.
The sall'allahu alayhis wa'as'salaam, which people referred to here is the variation by additional benediction to the alev ha shalom; which they created for Mohammad alone.
And, just to correct the error of someone who said they don't say the benediction to the departed, the non-Arabs do not follow the custom, but, the Arabs, whether they are Muslim or Christian, do use the benediction for all the mention of the departed ones.