Mudgalapurana 3.21.21

3.21.21

śastrādiviṣayogairye mṛtāsteṣāṃ mahāmune |
caturdaśyāṃ samākhyātaṃ śrāddhaṃ nānyatra śasyate ||

For those who died by weapons or the effects of poison, O great sage, / the śrāddha is declared to be on the fourteenth; it is not praised elsewhere.