I've read that the Sutta Nipata is one of the earliest collections of suttas and that two of its chapters, the Atthaka Vagga and one other (Parayana Vagga I think), are the oldest. The Wikipedia page has this:
Some scholars believe that it describes the oldest of all Buddhist practices.
2. Nakamura, Indian Buddhism, Japan, 1980; reprinted by Motilal Banarsidass, Delhi, 1987, 1989, pp. 45-6.
I don't have this book and I'm wondering why Nakamura thinks this is the case and what evidence he has. I'd also be interested in other scholars who make similar claims and their reasons for it.
EDIT: Corrected spelling and grammar.