Question
We know that the first person to enter Paradise is Prophet Muhammad . However, what about the people whom the Prophet   saw in Paradise and Hell when he ascended to the heavens during his 'Israa' (Night Journey) and Mi‘raaj (ascension to Heavens)? 
Fatwa
All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah, and that Muhammad   is His Slave and Messenger. 
The Prophet   never stated in any Hadeeth (narration) that there would be someone who would enter Paradise before he does on the Day of Resurrection. However, the Hadeeths stressing that some people have entered Paradise speak about the migration of souls to the afterworld. For example, in a Hadeeth on the authority of Bilaal, may Allah be pleased with him, when the Prophet   heard the sound of his shoes in Paradise, he asked Bilaal, may Allah be pleased with him, about the deed he had done that admitted him into Paradise before him . [Ibn ‘Asaakir]
According to Al-Haafith Al-‘Iraaqi   the Prophet  seeing Bilaal, may Allah be pleased with him, in Paradise before him   was in his dreams and did not refer to the Day of Resurrection. 
In reality, the Prophet   will be the first to enter Paradise. There is no contradiction because the case of Bilaal, may Allah be pleased with him, refers to the migration of his soul during his sleep. 
It should be noted that no person knows how this takes place, because it pertains to the Unseen and human senses are unable to perceive it. Therefore, we must believe in it and acknowledge it without trying to understand how. This is because our minds are too limited to comprehend such a matter. This is a true statement from the Prophet   in which we must believe. May Allah The Almighty guide us to what pleases Him.
And Allah knows best.