Praise be to Allah.
There is no difference of opinion among the scholars that laughing outside of prayer – even when one is getting ready for prayer – does not invalidate wudoo’.
Ibn al-Mundhir said: The scholars are unanimously agreed that laughing outside of prayer does not invalidate wudoo’ and does not require one to do wudoo’ again.
End quote from al-Awsat (1/166).
An-Nawawi said: They [the scholars] are unanimously agreed that laughing out loud outside of prayer does not invalidate wudoo’.
End quote from al-Majmoo‘ (2/61).
The scholars are unanimously agreed that laughing when praying invalidates the prayer.
Ibn al-Mundhir said: They are unanimously agreed that laughing when praying invalidates the prayer.
End quote from al-Ijmaa‘, p. 3.
What is meant by laughing here is laughing out loud
Ibn Qudaamah said: We do not know of any difference of opinion concerning that.
End quote from al-Mughni (2/451).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: Laughing out loud, in which one makes a loud noise, is contrary to the dignity of prayer and to the focus and humility that is required in prayer. Moreover, it shows disrespect towards the prayer, makes a mockery of it and is contrary to its purpose, so the prayer is spoiled by that.
End quote from Majmoo‘ al-Fataawa (22/617).
From the above, it is clear that the difference of opinion among the scholars has to do with laughing out loud in the prayer and whether wudoo’ is also invalidated by it, or not.
The correct view, which is that of the majority of scholars, is that it does not invalidate wudoo’, because there is no evidence to indicate that it does invalidate it.
Jaabir ibn ‘Abdillah (may Allah be pleased with him) said: If someone laughs whilst praying, he must repeat his prayer, but he does not have to repeat wudoo’. Narrated by al-Bukhaari in a mu‘allaq report, and by ad-Daaraqutni (51) in a mawsool report.
Ibn Hajar said: It is proven to be saheeh, but it is the words of Jaabir. End quote from Fath al-Baari (1/280).
Shaykh al-Islam said: There is no narration from any Sahaabi to suggest a different view. Moreover, it does not invalidate wudoo’ outside of prayer, so by the same token [it does not invalidate wudoo’] in the prayer.
End quote from Sharh al-‘Umdah (1/324).
An-Nawawi said: Our view and that of the majority of scholars is that it does not invalidate wudoo’. … This is the view of the majority of the Taabi‘een and those who came after them.
Al-Bayhaqi narrated that Abu’z-Zinaad said: I met some of our well-versed scholars whose view is decisive: Sa‘eed ibn al-Musayyab, ‘Urwah ibn az-Zubayr, al-Qaasim ibn Muhammad, Abu Bakr ibn ‘Abd ar-Rahmaan, Khaarijah ibn Zayd ibn Thaabit, ‘Ubaydullah ibn ‘Abdillah ibn ‘Uqbah, Sulaymaan ibn Yasaar, and many other respectable scholars, who say that laughing invalidates the prayer, but it does not invalidate wudoo’.
And he said: There is no proof at all to suggest that wudoo’ is invalidated by laughing.
End quote from al-Majmoo‘ (2/60).
The hadiths which say that laughing invalidates wudoo’ are da‘eef (weak) and none of them are saheeh (sound).
All of them are da‘eef and waahin (flimsy), according to the consensus of the scholars of hadith.
End quote from al-Majmoo‘ (2/61).
Summary: laughing does not invalidate wudoo’, whether that happens when praying or outside of prayer.
And Allah knows best.