pad is substituted for pāda before the affix śas when the meaning is that of a ṛk (Hymn).,
Thus पच्छो गायत्रीं शंसति = पादं पादं शंसति, the affix शस् being added by 5.4.43. Why do we say 'when meaning a Hymn'? Observe पादशः कार्षापणं ददाति ॥,
ऋचः शे
Adhyāya 6 · Pāda 3 · Rule 55