Hi Robert,
I knew it has to be in “The Ones” of AN because the quote states “Is there any single dhamma (…)”. Then I’ve searched for “appamād-” because I assumed that “heedfulness” stands for “appamāda”. It gave me 3 or 4 possibilities to choose from. I definitely don’t know suttas by heart! 
If you like to work with original Pāli I’d like to recommend Digital Pāli Reader for Firefox – it is a great tool with many helpful features.