When Muhammad the apostle of God reached the age of forty, God sent him in compassion to mankind “as an evangelist to all men.” Now God had made a covenant with every prophet whom he had sent before him, that he should believe in him, testify to his truth, and help him against his adversaries, and he required of them that they should transmit that to everyone who believed in them, and they carried out their obligations in that respect. God said to Muhammad, “when God made a covenant with the prophets [He said] this is the scripture and wisdom which I have given you, afterwards an apostle will come confirming what you know that you may believe in him and help him.” He said, “Do you accept this and take up my burden?” i.e. the burden of my agreement which I have laid upon you. They said, “We accept it.” He answered, “Then bear witness and I am a witness with you.” Thus God made a covenant with all the prophets that they should testify to this truth and help him against his adversaries and they transmitted their obligation to those who believed in them among the two monotheistic religions.

— from Sirat Rasul Allah (“The Life of the Messenger of God”), by Ibn Ishaq

Muhammad Ibn Ishaq

(d. 767) was an Arab Muslim historian and biographer who collected oral traditions that formed the basis of his biography of the prophet Muhammad. Orally dictated to his pupils, these traditions are now known as “Siratu Rasuli I-Lah” (“Life of the Messenger of God”) and survives in edited versions and fragments.