Where is the suttas is 'sati' defined as present moment awareness?
Sutta does not define that 'sati' as present moment awareness.
Mindfulness in Buddhist teaching is much more than the english meaning of mindfulness and the the general mindfulness taught in the west.
To understand the meaning of 'Sati' we should have a good understanding of the following two Suttas.

http://www.accesstoinsight.org/tipitaka/mn/mn.010.than.html

http://www.accesstoinsight.org/tipitaka/mn/mn.118.than.html