गवियुधिभ्यां स्थिरः

Operational rule vidhi

ष् is substituted for the स् of स्थिरः after the words गवि and युधि.

Word by word (padaccheda)

Left context गवि- युधिभ्याम् pañcamī · dvivacanaSubstitute / term स्थिरः prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Full reading

गवि-युधिभ्याम् स्थिरः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

ष् is substituted for the स् of स्थिरः after the words गवि and युधि।
Thus गविष्ठिर, युधिष्ठिर ॥ The compounding takes place by 2.1.44. and the 7th case affix is not elided by 6.3.9. though the word गो does not end in a consonant, yet it retains its Locative ending by virtue of this sutra.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.