Sources for times that one can daven that a sick person should die:
- Gemora Brochos 6a according to the version of the Gaonim
- Gemora Kesuvos 104a
- Gemora Taanis 23a (Choni Hamaagal)
- Gemora Bava Metzia 84a (Rabbi Yochanan and Rish Lakish)
- Sefer Chasidim: 519
- Ran Nedarim 40a