Mahāsutasomajātaka

Suttapiṭaka · Khuddakanikāya · Jātaka (2) · Vol. 28 · Section 381

[381] A person ought not to grant that boon which, even having granted it, he would not make good. Choose your boon, friend, without wavering — even at the sacrifice of my life, I will surely give 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English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.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 366 — Mahāsutasomajātaka