Quran 2:234

وَٱلَّذِينَ يُتَوَفَّوۡنَ مِنكُمۡ وَيَذَرُونَ أَزۡوَٲجً۬ا يَتَرَبَّصۡنَ بِأَنفُسِهِنَّ أَرۡبَعَةَ أَشۡہُرٍ۬ وَعَشۡرً۬ا‌ۖ فَإِذَا بَلَغۡنَ أَجَلَهُنَّ فَلَا جُنَاحَ عَلَيۡكُمۡ فِيمَا فَعَلۡنَ فِىٓ أَنفُسِهِنَّ بِٱلۡمَعۡرُوفِ‌ۗ وَٱللَّهُ بِمَا تَعۡمَلُونَ خَبِيرٌ۬٢٣٤

Those who die and leave wives, their widows shall wait four months and ten days (before they remarry). Once they fulfil their interim, you commit no error by letting them do whatever righteous matters they wish to do. ALLAH is fully Cognizant of everything you do.