Musnad Ahmad 528:8

Haywah ibn Shurayh narrated to us, Baqiyyah narrated to us, he said: Buhayr ibn Sa’d narrated to me, from Khalid ibn Ma’dan, from Jubayr ibn Nufayr, from al-Nawwas ibn Sam’an, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah has struck a parable: a straight path. On the two sides of the path are two walls, in which are open doors, and upon the doors are curtains. There is a caller calling at the beginning of the path, and a caller calling from above it. And Allah calls to the Abode of Peace, and He guides whom He wills to a straight path. As for the doors that are on the two sides of the path, they are the limits of Allah. No one falls into the limits of Allah until the curtain of Allah is lifted. And as for the one who calls from above it, he is the admonisher of Allah."