If a person praying alone breaks his Wudu during the Salah, is he supposed to finish his Salah with or without Salaam? What in case of a person praying with an Imam?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Anyone whose ablution is nullified during the prayer, his prayer is invalidated but he does not need to say Salaam or do anything that contradicts the prayer in order to get out of it. This is because the invalidation of ablution is considered as going out of the prayer because one of the conditions for the validity of the prayer is deficient. In fact, the prayer cannot be performed except in a state of purity.

So, anyone whose ablution is nullified during the prayer, is obliged to interrupt that prayer, perform ablution again, and then pray again a new prayer regardless of whether he was an Imaam, or is led in prayer, or is praying individually.

Allah Knows best.