Valmiki Ramayana Sundara Kanda Sarga 48 Shloka 39
Original Shlokaततोऽथ बुद्ध्वा स तदस्त्रबन्धं प्रभोः प्रभावाद्विगतात्मवेगः। पितामहानुग्रहमात्मनश्च विचिन्तयामास हरिप्रवीरः।।5.48.39।।
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ततः then, अथ and, सः हरिप्रवीरः that monkey leader, तदस्त्रबन्धंम् bond of that astram, बुद्ध्वा after realising, प्रभोः of the Lord, प्रभावात् by the grace, विगतात्मवेगः finding his speed arrested, आत्मनः for me, पितामहानुग्रहम् the boon of lord Brahma, विचिन्तयामास started thinking
The monkey leader realised that his speed was arrested by the power of the lord Brahma, by the strength of the bond of Brahmastram. He then recalled lord Brahma's boon to him.