There are different levels of being amused and each has its own ruling:

[1] "Tabassum", merely smiling, this will not break the Salah or a person's Wudu. One should however try avoiding this as it is against the manner in which this act of worship is to be performed.

[2] "Dahik", this is very light laughter that can only be heard by the person laughing himself. It will not break Wudu but will break the Salah.

[3] "Qahqaha", laughter that if there was a person next to you it could be heard by him. This extent of laughing will not only break the Salah but also that person's Wudu. The proof for this is:

A hadith narrated in the Mujam of Imam Tabarani. Abu Musa r.a. says "As the Messenger of Allah prayed a man entered and fell into hole that was in the masjid. The Messenger of Allah ordered all those who laughed to repeat the Wudu and the Salah." The narrators of this hadith are all reliable though the scholars have differed regarding some of them. (Iila al-Sunnan, vol.1 pg.158)

There are several other proofs mentioned in the multi-volume encyclopedia in Hanafi Fiqh "iila al-Sunna." One other proof worthy of mention is a hadith declared as Sahih by Imam Zafar Ahmad Uthmani and is found in the Sunnan of Imam al-Bayhaqi. Namely, Ubn Umar r.a. narrates that the Messenger of Allah said: "Whoever laughs aloud in Salah then let him repeat his Wudu and Salah."