This is a canonical quote, and it’s rather lovely. It’s from the Samyutta Nikaya, and in Bhikkhu Bodhi’s translation you’ll find it on page 708:
“We will develop and cultivate the liberation of mind by lovingkindness, make it our vehicle, make it our basis, stabilize it, exercise ourselves in it, and fully perfect it.” The Buddha Click To Tweet“Therefore, bhikkhus, you should train yourselves thus: ‘We will develop and cultivate the liberation of mind by lovingkindness, make it our vehicle, make it our basis, stabilize it, exercise ourselves in it, and fully perfect it.’ Thus should you train yourselves.”