Zuhri says: "(During prayer) laughing out loud or smiling does not break wudu." If what Zuhri has narrated from Hasan, from the Prophet Muhammad (peace be upon him), was authentic according to Zuhri, then he would not have given a fatwa contrary and opposite to it. And Allah knows best!
Hadith Referenceسنن الدارقطني / كتاب الطهارة / 621
Hadith Takhrij«أخرجه الدارقطني فى ((سننه)) برقم: 621، وعبد الرزاق فى ((مصنفه)) برقم: 3765»