Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 17: Brahmā Orders the Demigods to Incarnate and Assist Rāma
Text 1.17.7

पूर्वमेव मया सृष्टो जाम्बवानृक्षपुङ्गवः।
जृम्भमाणस्य सहसा मम वक्त्रादजायत॥

pūrvam eva mayā sṛṣṭo jāmbavān ṛkṣa-puṅgavaḥ
jṛmbhamāṇasya
sahasā mama vaktrād ajāyata

pūrvam eva = in the past; mayā = I had; sṛṣṭaḥ = generated; jāmbavān = Jāmbavān; kṣa-puṅgavaḥ = the best of the bears; jṛmbhamāṇasya = while I was yawning; sahasā = suddenly; mama = my; vaktrāt = mouth; ajāyata = he came out of.

In the past, I had generated the best of the bears, Jāmbavān. He suddenly came out of my mouth while I was yawning.

The demigods shouldn’t think that because Lord Brahmā is a deva himself, he should also generate a son. He had already done so in the past, as described here, considering that Jāmbavān would be an assistant to Lord Rāma in the future. The second half of this verse describes exactly how he was generated by Lord Brahmā.