Mindful  
Sata Sutta  (SN 47:35)

Near Sāvatthī. “Stay mindful, monks, and alert. This is our instruction to you all.

“And how is a monk mindful? There is the case where a monk remains focused on the body in & of itself—ardent, alert, & mindful—subduing greed & distress with reference to the world. He remains focused on feelings… mind… mental qualities in & of themselves—ardent, alert, & mindful—subduing greed & distress with reference to the world. This is how a monk is mindful.

“And how is a monk alert? There is the case where feelings are known to the monk as they arise, known as they persist, known as they subside. Thoughts are known to him as they arise, known as they persist, known as they subside. Perceptions are known to him as they arise, known as they persist, known as they subside. This is how a monk is alert.

“So stay mindful, monks, and alert. This is our instruction to you all.”

See also: SN 36:7