When the Messenger of Allah (may Allah's peace and blessings be upon him) missed prayer at night because of sickness or otherwise, he would pray twelve Rak‘ahs during the daytime
Authentic hadithعن عائشة رضي الله عنها قالت: كان رسولُ اللهِ صلى الله عليه وسلم إذا فَاتَتْهُ الصَّلَاةُ مِنَ الليلِ مِن وَجَعٍ أَوْ غَيْرِهِ، صَلَّى من النَّهَارِ ثِنْتَيْ عَشْرَةَ رَكْعَةً.
‘Ā'ishah (may Allah be pleased with her) reported: When the Messenger of Allah (may Allah's peace and blessings be upon him) missed prayer at night because of sickness or otherwise, he would pray twelve Rak‘ahs during the daytime.
Explanation
If the Prophet (may Allah's peace and blessings be upon him) missed the voluntary night prayer because of sickness or something else, he would make it up in the daytime. At night he would pray eleven Rak‘ahs as Witr, but since the Witr time passed, he would pray it as an even number, i.e. twelve Rak‘ahs instead of eleven.
Source · Narrated by Muslim
From the Hadith Encyclopedia (hadeethenc.com).