Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 31: Viśvāmitra, Rāma and Lakṣmaṇa Depart for Mithilā
Text 1.31.12

तद्धि यज्ञफलं तेन मैथिलेनोत्तमं धनुः।
याचितं नरशार्दूल सुनाभं सर्वदैवतैः॥

tad dhi yajña-phalaṁ tena maithilenottamaṁ dhanuḥ
yācitaṁ
nara-śārdūla sunābhaṁ sarva-daivataiḥ

tat = that; hi = indeed; yajña-phalam = was the result; tena = of a sacrifice; maithilena = conducted by the king of Mithilā; uttamam dhanuḥ = that excellent bow; yācitam = when the king pleaded for it; nara-śārdūla = O tiger among men; sunābham = it had a beautiful riser; sarva-daivataiḥ = and it was given by all the demigods.

O tiger among men, that excellent bow was indeed the result of a sacrifice conducted by the king of Mithilā. It had a beautiful riser and it was given by all the demigods when the king pleaded for it.

Why was that bow given during a sacrifice? That is answered here.