What is the sutta where the Buddha says that one breath or one bite of food is the proper timeframe with which to contemplate life/death?

I have heard this sutta referenced in dhamma talks by Ajahn Thanissaro, although I don't have a specific talk to point to. I know I have also read the sutta, but I can't remember when. In the sutta, various monks state that they practice by contemplating something along the lines of "If I live so long as one year... one month... one day... one hour... one breath... the time it takes to chew one bite of food... then I can accomplish a lot in the practice." The Buddha then states that the last two monks who cast their intention across one breath or one bite of food are heedful, while the other monks are not. Does anyone know which sutta this is? Thanks!