According to Islam, did God slay the firstborn children of the Egyptians during the plague of Egypt? I know that Jewish/Christian sources say this, but is this true according to Islam?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

There is no evidence in the Quran or the authentic Sunnah indicating that Allah, the Exalted, killed the firstborns of the Egyptians. However, some Muslim scholars quoted this from the Israelites, which is a source known to scholars as Al-Israa'eeliyyaat.

At-Tabari said in his Tafseer (Quran exegesis):

"Regarding the verse : {And We inspired to Moses, "Travel by night with My servants; indeed, you will be pursued."} [Quran 26:52], Ibn Jurayj said, 'Allah revealed to Moosa (Moses): ‘Gather the Israelites, the members of each four households in one house, then slaughter lambs and stain the doors of these houses with their blood, as I will command the angels not to enter a house that has blood on its door, and I will command them to kill the firstborns of the people of Pharaoh and of their properties. Then, bake flat bread, for it is faster for you. Then set out with those who worship Me (i.e. the Israelites) at night until you reach the sea, and My command will come to you then.’ Moses did as commanded. In the morning, Pharaoh said, ‘This is the work of Moses and his people! They killed our firstborns and those of our properties.’ He sent in their pursuit an army of a million and five hundred thousand, and five hundred kings each surrounded by a thousand men. Pharaoh went out escorted by his vast entourage and said as related in the verse: {Indeed, those are but a small band.} [Quran 26:54] Their number was six hundred thousand; two hundred thousand of whom were between the ages of twenty and forty.'"

Ibn Katheer wrote, "According to the reports of the People of the Book, when the children of Israel were commanded to go out of Egypt, Allah made that month the beginning of their year ... They said: On that night, Allah killed the firstborns of the Egyptians and the firstborns of their animals to distract them, and the Children of Israel went out safely..." [Al-Bidaayah wa An-Nihaayah]

Allah knows best.