The visarga of an ((is)) or ((us))-ending word, which is not preceded by any other word, is invariably changed to ((ṣ)) in a compound, when followed by a hard guttural or labial. Source: Aṣṭādhyāyī 2.0
Mahābhāṣya:With kind permission: Dr. George Cardona
1/29:anuttarapadasthasya iti kimartham | 2/29:paramasarpiḥkuṇḍikā | 3/29:atha idānīm anena mukte pūrveṇa ṣatvam vibhāṣā kasmāt na bhavati isusoḥ sāmarthye iti | 4/29:nānāpadārthayoḥ vartamānayoḥ khyāyate yadā yogaḥ |* 5/29:tasmin ṣatvam kāryam tat yuktam tat ca me na iha | See More
1/29:anuttarapadasthasya iti kimartham | 2/29:paramasarpiḥkuṇḍikā | 3/29:atha idānīm anena mukte pūrveṇa ṣatvam vibhāṣā kasmāt na bhavati isusoḥ sāmarthye iti | 4/29:nānāpadārthayoḥ vartamānayoḥ khyāyate yadā yogaḥ |* 5/29:tasmin ṣatvam kāryam tat yuktam tat ca me na iha | 6/29:vyapekṣāsāmarthye pūrvayogaḥ na ca atra vyapekṣāsāmarthyam | 7/29:kim punaḥ kāraṇam vyapekṣāsāmarthyam āśrīyate na punaḥ ekārthībhāvaḥ yathā anyatra | 8/29:aikārthye sāmarthye vākye ṣatvam na me prasajyeta | aikārthye sāmarthye sati vākye ṣatvam na syāt : sarpiṣ karoti |* 9/29:sarpiḥ karoti iti | 10/29:tasmāt iha vyapekṣām sāmarthyam sādhu manyante |* 11/29:atha cet kṛdantam etat tataḥ adhike na eva me bhavet prāptiḥ | yadi kṛdantam etat tataḥ adhikasya ṣatvam na prāpnoti |* 12/29:kim kāraṇam | 13/29:pratyayagrahaṇe yasmāt saḥ tadādeḥ grahaṇam bhavati iti | 14/29:vākye api tarhi na prāpnoti | 15/29:paramasarpiṣkaroti paramasarpiḥ karoti iti | 16/29:vākye ca me vibhāṣā pratiṣedhaḥ na prakalpeta | yat ayam anuttarapadasthasya iti pratiṣedham śāsti tat jñāpayati ācāryaḥ bhavati vākye vibhāṣā iti |* 17/29:atha cet saṃvijñānam nitye ṣatve tataḥ vibhāṣā iyam | atha avyutpannam prātipadikam tataḥ nitye ṣatve prāpte iyam vibhāṣā ārabhyate |* 18/29:siddham ca me samāse |* 19/29:ṣatvam | 20/29:kimartham tarhi idam ucyate | 21/29:pratiṣedhārthaḥ tu yatnaḥ ayam | anuttarapadasthasya iti pratiṣedham vakṣyāmi iti |* 22/29:nānāpadārthayoḥ vartamānayoḥ khyāyate yadā yogaḥ |* 23/29:tasmin ṣatvam kāryam tat yuktam tat ca me na iha |* 24/29:aikārthye sāmarthye vākye ṣatvam na me prasajyeta |* 25/29:tasmāt iha vyapekṣām sāmarthyam sādhu manyante |* 26/29:atha cet kṛdantam etat tataḥ adhike na eva me bhavet prāptiḥ |* 27/29:vākye ca me vibhāṣā pratiṣedhaḥ na prakalpeta |* 28/29:atha cet saṃvijñānam nitye ṣatve tataḥ vibhāṣā iyam |* 29/29:siddham ca me samāse pratiṣedhārthaḥ tu yatnaḥ ayam |*