Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 31: Viśvāmitra, Rāma and Lakṣmaṇa Depart for Mithilā
Text 1.31.8

तद्धि पूर्वं नरश्रेष्ठ दत्तं सदसि दैवतैः।
अप्रमेयबलं घोरं मखे परमभास्वरम्॥

tad dhi pūrvaṁ nara-śreṣṭha dattaṁ sadasi daivataiḥ
aprameya-balaṁ
ghoraṁ makhe parama-bhāsvaram

tat = that bow; hi = indeed; pūrvam = formerly; nara-śreṣṭha = O best of men; dattam = was given; sadasi = in a sacrificial assembly; daivataiḥ = by the demigods; aprameya-balam = of immeasurable strength; ghoram = terrible; makhe = in a sacrificial assembly; parama-bhāsvaram = and supremely brilliant.

O best of men, indeed, that terrible and supremely brilliant bow of immeasurable strength was formerly given by the demigods in a sacrificial assembly.

Where did that wonderful bow come from? This verse answers this question.