दीघ 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.