Subhūti Thera visits the Buddha with a companion, and the Buddha asks him who his companion is. Subhūti answers that he is the son of a believing disciple and has gone forth from a believer's home to homelessness. The Buddha then asks Subhūti if his colleague conforms to the traditional signs of the believer. Subhūti begs of the Buddha to define these signs, and the Buddha explains them at length. A.v.337f.
The Commentary says that the monk was the son of Anāthapindika and therefore Subhūti's nephew (AA.ii.865).