Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
quran:15:2 –

Perhaps those who disbelieve will wish that they had been Muslims.  

It may be that (read rubbamā or rubamā) those who disbelieve, on the Day of Resurrection, when they witness their predicament and that of the Muslims, will wish that they had been muslims (rubba here is used to indicate ‘many’, since many of them will wish this; but it is also said to indicate ‘few’, since they will be so stupefied by the terrors that they will not come out of their stupor to wish such a thing, except on a few occasions:
القرآن:١٥:٢ –

رُبَمَا يَوَدُّ الَّذِينَ كَفَرُوا لَوْ كَانُوا مُسْلِمِينَ  

{رُبَّمَا} بالتشديد والتخفيف {يود} يتمنى {الذين كفروا} يوم القيامة إذا عاينوا حالهم وحال المسلمين {لو كانوا مسلمين} ورب للتكثير فإنه يكثر منهم تمني ذلك وقيل للتقليل فإن الأهوال تدهشهم فلا يفيقون حتى يتمنوا ذلك إلا في أحيان قليلة.