here are different levels of being amused and each has its own ruling:
 "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.
 "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.
 "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:
1. The Prophet was leading a prayer and a blind man was walking and fell into a well, upon which some of the sahaba laughed while in prayer. When the prayer ended the Prophet ordered them to renew their ablution and then pray again.
2. The Prophet said, "Gurgulous laugh in prayer invalidates the prayer and the ablution."
If one laughs such that others could have heard your laughter, it invalidates both wudu and prayer.
If you laugh such that only you could hear it, then it does not invalidate your wudu but does invalidate your prayer. [Maraqi al-Falah, Durr al-Mukhtar Sharh Tanwir al-Absar]
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."