Hadithcore
Mishkat al-Masabih, 294

Jabir reported God’s messenger as saying, “The key of paradise is prayer, and the key of prayer is being purified." Ahmad transmitted it.

عَنْ جَابِرٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مِفْتَاحُ الْجَنَّةِ الصَّلَاةُ وَمِفْتَاحُ الصَّلَاة الطّهُور» . رَوَاهُ أَحْمد

References1 variant
In-Book Reference
Book 3, Hadith 13