Shloka 6.90.15
इन्द्रजित्-लक्ष्मणयुद्धम् तथा वानरप्रोत्साहनम् (Indrajit–Lakshmana Battle and the Rallying of the Vanaras)
एतान्निहत्यातिबलान्बहून्राक्षससत्तमान् ।बाहुभ्यांसागरंतीर्त्वालङ्घ्यतांगोष्पदं लघु ।।।
etān nihatya atibalān bahūn rākṣasasattamān |
bāhubhyāṃ sāgaraṃ tīrtvā laṅghyatāṃ goṣpadaṃ laghu ||
Having slain these many exceedingly mighty and foremost rākṣasas, and having crossed the vast ocean by the strength of your arms, cross this remaining obstacle too—small as a cow’s hoof-print—without effort.
"You have made short of many exceedingly mighty leaders of Rakshasas and swum the ocean. You cross this hoof -mark of a cow without delay."
The 'cow's hoof-print' represents a trivial or insignificant obstacle. The speaker contrasts this with the 'ocean' to tell the warrior that since they have already achieved the impossible (crossing the ocean/killing mighty demons), the remaining task is negligible by comparison.
Vibhishana uses the memory of past impossible feats—crossing the ocean and defeating great warriors—to instill confidence in Lakshmana, urging him to defeat Indrajit without hesitation.