I heard the Prophet ﷺ saying, "On the Day of Resurrection I will intercede and say, "O my Lord! Admit into Paradise (even) those who have faith equal to a mustard seed in their hearts." Such people will enter Paradise, and then I will say, 'O (Allah) admit into Paradise (even) those who have the least amount of faith in their hearts." Anas then said: As if I were just now looking at the fingers of Allah's Apostle.  

سَمِعْتُ النَّبِيَّ ﷺ يَقُولُ إِذَا كَانَ يَوْمُ الْقِيَامَةِ شُفِّعْتُ فَقُلْتُ يَا رَبِّ أَدْخِلِ الْجَنَّةَ مَنْ كَانَ فِي قَلْبِهِ خَرْدَلَةٌ فَيَدْخُلُونَ ثُمَّ أَقُولُ أَدْخِلِ الْجَنَّةَ مَنْ كَانَ فِي قَلْبِهِ أَدْنَى شَىْءٍ فَقَالَ أَنَسٌ كَأَنِّي أَنْظُرُ إِلَى أَصَابِعِ رَسُولِ اللَّهِ ﷺ  

The Messenger of Allah ﷺ as saying: He who has in his heart as much pride as much pride as grain of mustard-seed will not enter paradise. And he who has in his heart as much faith as grain of mustard-seed will not enter Hell. Abu Dawud said: Al-Qasmali has transmitted it from Al-A'mash in a similar way.  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ يَدْخُلُ الْجَنَّةَ مَنْ كَانَ فِي قَلْبِهِ مِثْقَالُ حَبَّةٍ مِنْ خَرْدَلٍ مِنْ كِبْرٍ وَلاَ يَدْخُلُ النَّارَ مَنْ كَانَ فِي قَلْبِهِ مِثْقَالُ خَرْدَلَةٍ مِنْ إِيمَانٍ