Is it permissible to say "Allahumma laka al-hamd (O Allah, to You be all praise)" after sneezing, or is it obligatory to say "Alhamdulillah (All praise is due to Allah)"?

The first is to say: "Praise be to Allah (Alhamdulillah)" when sneezing, because following the exact words of the Prophet, peace and blessings be upon him, is more complete and better. For remembrances (dhikr) are specific and their wording should not be altered. Al-Bukhari narrated that the Prophet, peace and blessings be upon him, said: "When one of you sneezes, he should say: 'Praise be to Allah (Alhamdulillah).'"