मज्झिम निकाय 2:85
SUJATOIti ajjhattaṁ vā vedanāsu vedanānupassī viharati, bahiddhā vā vedanāsu vedanānupassī viharati, ajjhattabahiddhā vā vedanāsu vedanānupassī viharati;
And so they meditate observing an aspect of feelings internally, externally, and both internally and externally.