Looking for citation for a quote

Hi! Can anyone help me find the oldest or most famous source for the idea that "all beings have been your mother countless times"? Not sure if this occurs in Buddhist scriptures, but I've been told that it does. Can't seem to find it, does anyone know?

Thank you!

Ernie

Oldest is problematic because dating is slippery and most famous is subjective.

But, still, for relatively old and famous, the germ of the idea is in Nāgārjuna's Suhṛllekha (Letter to a Friend), sloka 68.
- English translation.
- Another with Tibetan transliteration.
- (Tibetan version of) Sanskrit transliteration.

Thank you, MMcM!