Mahāsutasomajātaka

Suttapiṭaka · Khuddakanikāya · Jātaka (2) · Vol. 28 · Section 366

[366] That boon a person ought not to give which, having given, he would then revoke. Choose, friend, without wavering: even at the sacrifice of my life, I will surely grant it.

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.

This passage also appears in 1 other place

Vol. 28 · Section 381 — Mahāsutasomajātaka