The Bodhisatta was once an ascetic. He was born into a brahmin family and as he started to amass some personal wealth he fell dreadfully sick with jaundice. No doctor could help him, and as he and his family worried he might die he promised that if he got well he would live a religious life. Eventually he took a medicine that worked and he recovered. He kept his promise and went off to the Himalayas to live as an ascetic and was never happier.
In the Lifetime of the Buddha
A man fell sick with jaundice and doctors thought his case was hopeless. The man vowed that if he was cured he would live a religious life. A few days later he took a medicine that cured him and so he became a disciple of the Buddha, and eventually an arahant.
When the Buddha heard some disciples discussing it, he told them this story so they knew that he himself had done the same thing in the past.
The Buddha did not identify any earlier births other than his own.