Gethin (1998), pp.186-187; and, Rhys Davids & Stede, op. cit.
buddhanet.net
Sn-A 128 (cited by Rhys Davids & Stede, 1921–25, op. cit.); see also, BDEA & BuddhaNet (n.d.). Similarly, the post-canonical Visuddhimagga, chapter IX, vv. 105-109, provides further elucidation, such as with a metaphor describing mettā as a mother's wish for her (healthy) child to grow up and karuṇā as a mother's wish for her sick child to get well (Buddhaghosa & Nanamoli, 1975/99, pp. 313-4).