Importance of Casting Asthi in Ganga

The Padma Mahapurana, Section 1 – Sṛṣṭi-khaṇḍa (section on creation), Chapter 62 – In Praise of the Gaṅgā

यावदस्थि मनुष्यस्य गंगातोये प्रतिष्ठति ।
तावद्वर्षसहस्राणि स्वर्गलोके महीयते ॥ ६६ ॥
पित्रोर्बंधुजनानां च अनाथानां गुरोरपि ।
गंगायामस्थिपातेन नरः स्वर्गान्न हीयते ॥ ६७ ॥
गंगां प्रतिवहेद्यस्तु पितॄणामस्थिखंडकम् ।
पदेपदेश्वमेधस्य फलं प्राप्नोति मानवः ॥ ६८ ॥
धन्या जानपदा ये च पशवः पक्षिकीटकाः ।
स्थावरा जंगमाश्चान्ये गंगातीरसमाश्रिताः ॥ ६९ ॥

yāvadasthi manuṣyasya gaṃgātoye pratiṣṭhati |
tāvadvarṣasahasrāṇi svargaloke mahīyate || 66 ||
pitrorbaṃdhujanānāṃ ca anāthānāṃ gurorapi |
gaṃgāyāmasthipātena naraḥ svargānna hīyate || 67 ||
gaṃgāṃ prativahedyastu pitṝṇāmasthikhaṃḍakam |
padepadeśvamedhasya phalaṃ prāpnoti mānavaḥ || 68 ||

Till such period that the bone (अस्थि) remains of a man stands in the water of river Ganges, even for thousands of years, the man will be honoured in heaven. Person whose bones (after death and his body is cremated as per Hindu rites) are immersed in Ganga waters( in the river Ganga) He will be exalted to Swarg Lok(Hindu Concept similar to but not same as Heaven) for thousand years. A man, by casting (i.e. who casts) the bones of his parents, his helpless relatives or even of his guru, does not fall from heaven. If a man mixes the bone remains of his parents, relatives, orphans and guru in the waters of Ganges, then he will not get a world that is lower than heaven. The man, who would carry the piece of the bone of his parents to Gaṅgā, obtains, at every step, the fruit of a horse sacrifice. If a person bears the bone remains of his parents, then he yields the fruit of Aśvamedha yajña for his footsteps towards the river.