Hadith 1:36
Isnād Da'īf (Zubair `Aliza'i)
Mu'adh b. Jabal reported that God’s messenger said to him, “The keys of paradise are the testimony that there is no god but God.” Ahmad transmitted it.
عَن مُعَاذِ بْنِ جَبَلٍ قَالَ: «قَالَ لِي رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مَفَاتِيحُ الْجَنَّةِ شَهَادَةُ أَنْ لَا إِلَه إِلَّا الله» . رَوَاهُ أَحْمد
Original Reference: Mishkat al-Masabih 40