According to Amy's video
(found under 'Free Videos' and 'tips' the rule is knitwise if one is
decreasing and purlwise if one isn't
decreasing. So in your case it would be knitwise.
As you will end up with a left leaning knitted decrease (going from 3 stitiches to 1 stitch) between 2 purl stitches I would move the yarn back before sliping the stitch to give a neater finish.