Siṅgālasutta

Suttapiṭaka · Dīghanikāya · Pāthikavagga · Vol. 11 · Section 194

[194] Householder's son, the friend who shares in happiness and suffering should be known as a good-hearted friend on four grounds: he reveals his own secrets to him; he keeps his friend's secrets concealed; he does not abandon him in misfortunes; he would sacrifice even his life for his friend's sake. Householder's son, on these four grounds the friend who shares in happiness and suffering should be known as a good-hearted friend.

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.