Sunan Ibn Majah Book 1, Hadith 612

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا حَمَّادُ بْنُ خَالِدٍ، عَنِ الْعُمَرِيِّ، عَنْ عُبَيْدِ اللَّهِ، عَنِ الْقَاسِمِ، عَنْ عَائِشَةَ، عَنِ النَّبِيِّ ـ صلى الله عليه وسلم ـ قَالَ ‏ "‏ إِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ نَوْمِهِ فَرَأَى بَلَلاً وَلَمْ يَرَ أَنَّهُ احْتَلَمَ اغْتَسَلَ وَإِذَا رَأَى أَنَّهُ قَدِ احْتَلَمَ وَلَمْ يَرَ بَلَلاً فَلاَ غُسْلَ عَلَيْهِ ‏"‏ ‏.‏

It was narrated from 'Aishah that:The Prophet said: "If anyone of wakes up and sees some wetness, but he does not think that he had an erotic dream, let him have a bath. But if he thinks that he had an erotic dream but he does not see any wetness, then he does not have to take a bath