Musnad Ahmad 25:1531

Isma'il narrated to us, Ayyub narrated to us, he said: I was informed from Sa'id ibn Jubayr, he said: Ibn Abbas said: The angel came with it, until he reached the place of Zamzam, and he struck with his heel, so a spring gushed forth. The woman [Hajar] hastened and began to scoop [the water] into her waterskin. The Messenger of Allah, may Allah bless him and grant him peace, said: "May Allah have mercy on the mother of Ishmael; had she not hastened, Zamzam would have been a flowing spring."