A doubt on tritiya vibhakti and the applicable sutra

[Vikram Majumdar]

नमो विद्वद्भ्यः 

Can anyone please clarify which sutra of Ashtadhyayi mandates the Tritiya vibhakti in the pada "रूपेण" in usages like in the following verse and in other places:

या देवी सर्वभूतेषु शक्ति-रूपेण संस्थिता। नमस्तस्यै नमस्तस्यै नमस्तस्यै नमो नमः॥

Regards

Vikram Majumdar